The entrance to the studios of Rinse FM, just off the courtyard of the now-defunct bar and venue 93 Feet East, is an unprepossessing affair. An unmarked door below an iron fire escape opens onto a flight of stairs, and the only giveaway that you’re somewhere with any cultural relevance is the occasional old poster for a club night or a compilation CD curated by the station, blue-tacked to the wall. For all Rinse’s colossal musical influence in the past 15 years, and growing success since going legal in 2010, there isn’t even a sign above the door; as radio station record labels go – and not just ones that have played pivotal parts in steering the passage of British electronic music since grime crawled out of jungle in the late ’90s – it’s not one to show off.

At the top of the stairs is a non-descript room that feels more like the ever-changing council flats that the station used to call home than the beating heart of hipster East London. From a studio next door comes the muffled sound of a track being mixed: a low-level thump throbs along the floorboards, sporadically stopping and restarting; every now and then, the door cracks ajar just enough for the rest of the tune above it to jump out. It’s the only remarkable thing about the entire scene, and that detail makes Rinse’s message clear: like so many pioneering musical forces before them, their only concern is for the music, maaan – and the pop-star accoutrements of style and champagne, commercial adoration and Scrooge McDuck swimming pools can go hang.

All of which makes Rinse’s most successful charge, Kathleen Anne Brien – known to everyone but her mum as Katy B – such a curious prospect. Tonight, fresh from the adjacent session and dressed in jeans, printed sweater and some high-tops, she cuts a figure in keeping with her employers’ aesthetic. But alongside that, Brien is also a bona fide pop star, with top-ten singles, a face that gets stopped in the street and a performance CV that lists Radio 1’s Big Weekend and countless national TV slots. A pop svengali’s dream, she’s happy singing over other people’s songs, collaborating with behind-the-scenes songwriters, coming up with killer pop hooks and doing it all in front of screaming 14-year-olds. Not for Brien the noble struggle against the artistically corrupt mainstream, or desperate drive to make wilfully odd music: Katy B is the BRIT School-educated singer who cites Destiny’s Child more readily than Diplo, and makes no apology for it. “I listen to Radio 1, who like to play songs,” she says, emphasising the last word. “And I love songs. I’m not trying to make really serious, leftfield music – like I’ve said before, I’m a massive Justin Timberlake fan, and I love R’n’B. Pop music is a massive part of me anyway – it never wasn’t. It was always going to be half of my influences.”

But it’s the other half of Brien’s influences that intrigues: here’s a singer who began her performance life as a funky house hype girl, singing over tunes on pirate radio and in dark, underground clubs, a world away from the Mickey Mouse Club production line or the micromanagement of Simon Cowell. For all her love of chart pop, you get the impression that if you cut Brien, she’d bleed dubstep, UK garage and house: she talks with breathless enthusiasm about her formative experiences in the London club scene over the last eight or so years, and comes across as both an authority and wit on everything dance music – from the splintering of genres to the dancing itself: “early dubstep was very sparse, very masculine… proper sausage dancing,” she observes with a nostalgic chuckle. “And all these girls at funky nights would be dressed up in these tiny little skirts and amazing heels and would be dancing – skanking! – hard!” From a certain angle, her pop chops seem like a distant pinprick.

Three years ago, this pop/pirate musical hybrid fashioned her debut album out of a series of tracks originally destined to be a showcase sampler for Rinse’s stable of rising producers. Brien’s initial role on the record was simply to sing anonymously over the top, like she had done so many times on the radio, but her vocals made it quickly apparent that the tracks deserved to be more than just a calling card for the likes of Geeneus, Benga and DJ Zinc. Accordingly, ‘Katy B On A Mission’ was born, adding an approachable high-street gloss to dance music’s murky subcultures. While the Rinse logo on the back of the record made sure the street credibility endured, ‘On A Mission’ was also deeply indebted not just to the slick American pop and Timbaland R’n’B that Brien loves, but also the ’90s commercial pop-house of Steve ‘Silk’ Hurley and Kim Syms. Almost by accident, Rinse FM had an unapologetically accessible mainstream dance record on their hands. Without batting an eyelid, their next release was an EP of minimal and sonically challenging UK bass; normal service was, for the label at least, resumed.

‘On A Mission’, though, turned out to be something of a landmark record: one only need look at the recent successes of Jessie Ware, AlunaGeorge and Disclosure, or even someone as environmentally different as Miley Cyrus, to see how well Brien’s original formula – pirate-radio beats meets Capital Radio melodies – has been refined, to chart-topping effect. Not that Brien was expecting it: “I didn’t for a second think it would take off like it did,” she admits, looking back. “When it started happening I was on holiday with my friends, and I didn’t even turn my phone on – I came home and the single had had 50,000 views on YouTube, which was really strange.

“I remember at the time thinking that there wasn’t anyone else doing the same kind of thing, which is why I didn’t think it was going to do much,” she adds, with refreshing honesty. She has a point too: the biggest selling singles of 2010 ranged from the guile-free bubblegum of Katy Perry to joyless, po-faced Jason Derulo/Flo Rida jams, and pretty much anything featuring Rihanna. “I didn’t write it for me to make money off,” admits Brien. “I just wrote those tunes for what I was doing at the time – singing my songs, living at home, going out clubbing every weekend with my mates and getting paid my fifty quid or whatever for a PA at a rave.”

But when ‘On A Mission’ got to number two in the charts, went gold and earned a Mercury Prize nomination, there were inevitable tuts about authenticity, despite Brien’s considerable clubbing experience. Coinciding with Rinse FM becoming a legally regulated radio station and the opening flurries of the dubstep-pilfering Americanised EDM in the charts, the argument from the purists was that ‘On A Mission’ was just another symptom of the whole scene coming above ground and, with that, losing its soul. Brien admits to being as unprepared for the backlash as she was for success: “When I was making the album, I thought maybe I could just have a flower or a cartoon character on the front cover, because all the other artists on Rinse were a bit more culty, a bit more anonymous,” she explains, making something of an understatement about a group of producers with names as transparent as Oneman, Roska and Royal-T. “And I’m not going to lie, I think I did find the whole thing with people knowing who I was quite difficult. I’d always dreamt of being a singer, but I’d never really thought about putting yourself out there on that level to let people have an opinion on you. It was really quite daunting.”

Not for the last time, perhaps, Brien felt the opposing pulls of commercial success and underground credibility. “People were talking about how it was selling out dubstep or whatever,” she remembers of the reaction to ‘On A Mission’. “But it’s not selling out anything, because I’ve been in them raves and that means something to me. That’s something that’s sacred to me. I wrote those songs for the rave. It’s something that I feel really passionately about, and I think that’s what, in everything I do, I try to keep. When you’re writing a club record, you want to smash the rave. You want to smash it, you want it to have the best feeling, you want to make people dance, you want it to stand out.”

Of course, the only problem with “writing for the rave” is that to keep it fresh you’ve got to keep on being a raver, which, when you’re Katy B, is increasingly difficult. “You know when you have to go to bed or stay in because you’ve got something important the next day?” asks Brien, longingly, explaining her decreasing clubbing attendance in recent years. “Well, my life now is something important the next day, every day – a TV performance or photo shoot, or I’ve got to be up to record a vocal and I really want my voice to be on point.” There’s a resigned raise of the eyebrows with this, and while it would appear that her success doesn’t seem too compatible with her love of staying up all night, Brien’s remaining practical, at least outwardly: “To be a singer, and to be in this kind of environment, you have to be very dedicated,” she says, although not sounding convinced of herself. “You have to work really hard,” she continues, almost bored of her own pragmatism. “There’s so much sacrifice that you have to make – and I guess staying in is part of it.”

But the joy of dancing until dawn also appears at odds with Brien’s ever-encroaching adulthood. Whether she likes it or not, she and her friends are beginning to settle down, cosy up and maybe prefer the more pedestrian side of being in your twenties: “I’ve moved out of my mum and dad’s house and have had to pay the bills!” she explains with mock exasperation of her recent life. “My friends have to go home at a sensible time on a Saturday night, and I have to go home too, because I’ve got a career now rather than a Saturday job.”

Crucially, however, these different flavours of new responsibility all add up to an enforced maturation that has affected Brien’s songwriting. While only 24 – certainly not an age at which one should be longing to be young again – her new material has undeniably darker themes than the frequently straight-up “wooo drunk” of her debut, which speaks perhaps of a newfound thoughtfulness and brooding. “As I’ve got older, after I’ve been raving with my friends, I’ve come home and felt a bit more alone,” she acknowledges, explaining her latest single ‘5am’, which vividly tells the story of a post-club comedown. “You know the hangover’s going to be worse in the morning, and you’re by yourself in your flat, and because you’ve been having such a nice time, you suddenly just feel a bit lonely, you know?” The single is by no means a comedown tune though: despite Brien’s clubber suffering from the horrible affliction of being wide awake while desperate to sleep, the song is set to disconcertingly peppy house beats and banging synths, and the result is something slightly queasy.

The same affect is writ even larger on the best of her new batch of songs, ‘Crying For No Reason’. Lest the title not be indicator enough, things aren’t exactly rosy for the song’s protagonist: “My friend called me up one evening crying,” explains Brien of the tune’s inspiration. “She’d stopped at the traffic lights and just suddenly started balling her eyes out and didn’t know why. Thing is, she’d seemed fine for ages, but underneath she wasn’t. She’d tried to tell herself for so long that she was okay, but then this day just came, and everything got on top of her, and I can really relate to that.”

It’s a brutally direct track, and in the context of the existing Katy B persona, with song after song about euphoric clubbing with your mates, an indirect acknowledgment of having serious waves of depression is quite startling. Thankfully though, Brien is candid about her vulnerability, and pleased to get it off her chest: “I’m glad that I got the opportunity to write something like that,” she says of ‘Crying For No Reason’, sounding shyly defiant, “because I think some people think I just write about raving with the lights on all the time and that my life is just this one big happy party. But really, for me, I mean, I have these crazy dreams at the moment that I can’t even talk to you about, so going out is what I do sometimes because it makes me happy – and because sometimes in my life, when I do feel a bit lonely or do feel a bit sad, going out is my escape.”

She trails off, suddenly sounding exposed, as if songs about loneliness and psychological pain aren’t very her. Could it be that if ‘On A Mission’ was the fun of growing up, her new songs tackle the more real-life pressures of actually being grown up? “I’d say so – well, they try to,” she concedes, explaining the transformation. “I think with ‘On A Mission’, I was 18 to 21 and I was just on the cusp of being an adult. All your life, from when you’re born, you’re chasing to be an adult – you’re chasing to get into a club, you’re chasing to have your first boyfriend or to be in love, and I was experiencing all those things – my first boyfriend and real relationship, I was going out, I could do whatever I wanted to. My mum wasn’t saying that I had to be back in at a certain time, and there was a level of freedom on that record that spoke of being in love with life and the whole world. And I think that this new record…” She trails off again. “Well, I’ve had another relationship since then and that was very like…” Another trail. “Basically, I had had a break-up where I probably felt similar issues [to my friend from ‘Crying For No Reason’]. That person could’ve been the person that I spent the rest of my life with.”

The song itself, too, is refreshingly candid in its own way. Where a rock musician trying to convey psychological trauma has a long canon from which to draw inspiration, in Brien’s position as a pop-dance singer, the landscape for songs about mental breakdowns is slightly fuzzier, leaving her room to improvise. With that freedom she opts, rather unexpectedly, for something not unlike Robert Miles’ 1995 prog-house hit ‘Children’: a delicate piano opening gives over to hulking trance synths and tumbling, echo-slathered drums, leaving a track full of muscular melancholy destined to accompany footage of crestfallen sports stars suffering valiant defeat, or the tears of ousted X-Factor hopefuls. While functional, it’s not exactly the most forward-thinking production to come out of Rinse FM, and would be a decidedly cheesy mess were it not for Brien’s delivery, which not just saves the track but elevates it to something really rather compelling. Rising quickly from an initial whisper to a full-blown torch-singer howl, her vocal may be from the more direct school of performance, but it’s no less effective for it.

What’s most striking about ‘Crying For No Reason’ though is that it seems to symbolise Katy B’s evolution from hype girl to centre stage: the emphasis is now on her songs, which happen to have house and UK garage production, rather than the other way round. Perhaps most tellingly, while Brien has retained the services of long-standing producer Geeneus, she needn’t have. Perhaps it’s a symptom of the sound of her debut being so much more ubiquitous two years on, or perhaps it’s just Brien herself moving on organically, but if the rest of her new material follows in the same vein as ‘Crying For No Reason’ – only five new tracks have been made available to the press, all of them in “demo” form – then Brien could be embarking on a journey that would take her far away from her natural habitat, something about which she’s in two minds: “I’m happy to be part of the Rinse family,” she says unthinkingly, but then softens: “But I guess there will come a time one day when I have to be on my own. The only thing is that I’m used to making music with people who I get on with. I think you have to have a social connection with someone when you’re making music, and when I make music with new people, I feel quite shy. Meeting a new producer is like meeting your boyfriend’s parents for the first time!”

“Then again,” she adds, glancing around the homely Rinse studios she knows so well, “with this lot I’m completely in my comfort zone, and I think it’s good to step out of your comfort zone, so I’m cool to do that. I’m up for that some day.”

Quite when that day might be, though, is open for conjecture. ‘Little Red’, Brien’s follow-up to ‘On A Mission’, was originally due out in Spring 2013 but as of November remains unfinished and is now slated for a release next February. “I don’t know what’s going on with it,” says Brien, apologetically, when quizzed about the year of delays. “The songs are finished, and we have an album, but it’s just being tweaked. It’s all about the politics of what’s going to be the single, and what’s going to do this that or the other, apparently.

“It’s frustrating when you have songs and you want people to hear them, definitely, but it’ll be out soon – and I’m patient,” she insists, “I’m fine.” Then she realises how bad a liar she is: “Not really actually. I’m not patient at all! It’s a skill I’m having to learn!”

In terms of acquiring new skills, Brien has done a lot in the past two years to deal with her own growing pains and increased public exposure, and it’s a tribute to her individuality that she retains a maverick streak. “My managers tell me off because I’ll be in Brixton McDonald’s at four in the morning and they’ll be like ‘what are you doing?!’” she says of her attempts to continue living a normal life. “But I’m quite small, so I just put my hood up and I’m fine.” Equally, where many singers in Brien’s position would be all too keen to knock out an ‘On A Mission Volume 2’, her interest in tackling her own demons makes for absorbing listening.

But for all the maturity, it’s nonetheless easy to see Katy B as being at a slight crossroads. As a member of the post-everything generation, for whom access to the darkest recesses of club culture are just as many clicks away as Beyoncé, and for whom the very idea of genre purity is a nonsense, her affinity for Rinse and all things underground makes as much sense as her fondness for earth-conquering pop. But what is perhaps a more difficult circle to square is that the prolonged carefree adolescence that indirectly made Brien so successful has been replaced, albeit perfectly naturally, by a young-adult introspection. “When I was working on my funky house or whatever,” she says, earnestly, “I was thinking ‘what will make the girls really sing along to this passionately, like really connect to it’.” Her new subject matter, though, is more pensive, and although self-reflection has proved a rich seam to mine for everyone from Madonna to Blur, it makes for a curious and not always comfortable fit with the forward-thinking club music to which she clings so fondly.

Currently, the fact that she does cling to it makes Katy B a fascinating artist, full of contradictions and paradoxes, and a writer of terrifically addictive songs that on closer inspection are pleasingly odd. It’s clear however that she would also make a brilliant pop singer – something of which she’s perfectly aware: “I definitely feel like I have to embrace being a pop star somehow,” she says, reluctantly. “If you’re going to do something, you have to crack on with it properly.” Of course, it’s another matter entirely whether or not she will.