Inside a flat somewhere in north London, the tough beats of a succession of UK garage anthems are blasting out from the only bedroom. Drawn blinds keep out the gloom of a wet and miserable Saturday afternoon, while a darkened figure prepares his next mix on the set of decks lining one of the walls. It all looks like the standard bedroom DJ set-up, but as the guy on the decks begins to speak into the mic it becomes clear he is not just playing for the practise.

"All-right people, keep it locked," he announces, "You're listening to the Naturalist, sounds of the Deja, 92.3. We're back."

It might not be a glamorous location, but this flat is home to Déjà vu, one of the biggest pirate radio stations in London. It's taken us weeks to get up here after a succession of false starts (we got to Déjà vu's last temporary home only to find they were about to be raided) and we spent several days chasing around dodgy London council estates. Today's visit was nearly called off as well, and if the Naturalist and his friend JD look a little bit ragged, it's with good reason: they spent last night in police cells after they ran into some problems setting up the station's transmitter. Welcome to the underground.

According to the Department Of Trade and Industry, whose job it is to police the airwaves, there are hundreds of pirates broadcasing in London, and their numbers are increasing nationwide. Many are small-time, amateurish outfits that fade away as quickly as they spring up, but others, like Déjà vu, manage to negotiate the perils of illegal broadcasting long enough to become established figures on the radio dial. They may not be household names like Kiss or Radio 1, but to an army of dedicated listeners, they mean more than the big-time legal stations could ever do.

Kiss are always three years behind and Radio 1 are five years behind

There are pirates covering most forms of music neglected by the mainstream, from soul and reggae to drum'n'bass. Déjà vu specialises in the UK garage sound that has swept through London in recent years, dominating the capital's clubs and airwaves in equal measure. The station has been running for over two years and DJs like Mike 'Ruff Cut' Lloyd, Wayne Pacha, Pied Piper and the Naturalist have helped it earn a reputation as one of the most respected garage pirates around, although it hasn't been an easy ride. Running a pirate isn't without its problems and it sounds like Deja have been having more than their fair share recently. Like last night, for instance, explains the Naturalist, an infectiously cheerful 25-year-old from Hackney in the East End.

"I suppose you could say we came unstuck a bit," he laughs as he takes a break from his set. "Which is why I'm not feeling too good today. We had to come back on today though, even with that little set-back, because we haven't been on for the last few weeks and we've got events coming up." As well as Djing on the station, the Naturalist is also Deja's engineer. This means plenty of sleepless nights on top of tower blocks making sure transmitters and aerials are working properly, as well as the risk of getting caught by the authorities. Last night it was only the police who caught him, and as illegal broadcasting is not a police matter he'll probably get off with a caution. He would've been in more trouble if it had been a special unit from the DTI.

The DTI are the main obstacle pirate radio stations face in the struggle to stay on air. As well as tearing down aerials and taking transmitters, they occasionally raid studios and confiscate any equipment they find, including records. Anyone caught by them can expect a hefty fine, but that's only the start of a pirate engineer's worries. Rival stations sometimes steal equipment from successful pirates; nosy neighbours can call the police if they hear suspicious noises on the roof, as they did last night; and if anything goes wrong with a transmitter, someone has to be on call to put it right. It all sounds like a lot of hassle to go through, so what is it that makes pirate radio so worthwhile for the duckers and divers involved?

"I just love radio, I get a buzz from doing it," replies the Naturalist. "Even now, when I'm nearly falling asleep, as soon as the phoneline goes it wakes me up, because you think, yeah, loads of people are listening. I do it to get myself out there as well, but its mainly for the buzz." The phoneline hasn't gone off for a while and the Naturalist realises why when he sees that his mobile is switched off. As soon as it's turned back on, the shouts start coming in thick and fast, with calls from the Edware Road Posse, the Canning Town Massive and even some obscure town in Kent. The Naturalist is wicked on the mic, with a free-and-easy conversational style all of his own. Vicas from Main Source, who put us in touch with Déjà vu in the first place, got himself a job answering the phone, although he doesn't seem too bothered. Apparently it's "pure babes" calling. Considering what happened last night and the illicit nature of what's going on, the atmosphere in the studio is incredibly relaxed. The Naturalist is spinning a top-notch UK garage set, with soulful vocals and sweet melodic riffs contrasting with hard, snapping beats. The latest tunes from Dreem Teem and MJ Cole get a run-through, and Christine Blonde's Love Shy gets a repeat spinning after a request from a female caller. "The ladies love that tune," comments the Naturalist.

At first it's exciting because it's something new, but then it becomes like a drug and you can't do without it

In an adjoining room, Joanne, Déjà vu station manager and the only lady in the flat, is nodding her head appreciatively, so maybe he's right. Joanne founded Deja around two-and-a-half years ago with three other partners. The others left, but by that time she had become addicted to pirate radio and she decided to carry on on her own. "At first it's exciting because it's something new, but then it becomes like a drug and you can't do without it," she explains. When everything goes right and you can hear yourself come on air, it's just a brilliant feeling. There are problems that go with it, but that's just part of it, and you learn to put up with them. Sometimes I wish things could run a bit more smoothly though."

It's easy to see why. The cost of equipment being confiscated and stolen means it is difficult for a pirate to turn over a profit from advertising alone, so like most other stations, Déjà vu promotes raves and club nights to raise more money. But while she envies the security of legitimate broadcasters, Joanne feels that Deja could never operate as it does under the restrictions that legal stations face. "It would be very, very hard for us to go legal,"she claims. "Even if we were allowed to broadcast legally, we wouldn't be able to because of all the licensing laws. A lot of the tunes we play are dub-plates that DJs have made for themselves, and we couldn't do that if we were legal because no-one would know where the licensing money was meant to go. It's a silly system, really, because this is the music that's happening and no-one can play it except us."

Joanne believes the current system means that there will always be a place for pirates to push through new music and represent tastes not covered by legal radio. And on past evidence, it's difficult to argue with her. From the soul and reggae pioneers of the '80s, through the house and hardcore pirates of the early rave boom, to the jungle stations of the mid '90s and onto the garage explosion of the present day, pirate broadcasters have been instrumental in the development of the most exciting forms of dance music to emerge from the capital in the last 15 years. Admittedly, most stations have a limited shelf-life, but Joanne's confident about Deja's future. "Hopefully we'll still be around in five years time - unless the authorities make it impossible."

Back in the studio, the Naturalist is passing the headphones to the next DJ as his set finishes. An MC arrives looking every inch the part in black clothes and Versace shades. He chats over some particularly tough dubs and dances around the room, head-bobbing to the music. Chilling out next door the Naturalist gives me his thoughts on the UK garage scene. He compares it to the jungle scene a few years ago and thinks it could still become popular, exploding to national popularity just as jungle did. "Back then you had a buzz about it with Rush, Infection and all those stations, and I think it's the same now," he says. "I can see it spreading out to the rest of the country because it's the sound of the underground, the sound of now, and people will want a piece of it. I can see the pirates springing up all over, if people from London with the know-how go out there and show people how it's done, he reasons. "It would work because the airwaves in London are crowded, but there's no-one doing it out there."

Whatever the future of the garage scene, the Naturalist is certain that pirate radio will have a part to play in it. Without pirates, he claims, UK garage might not even exist. "It definitley would never have come through like it has. Stations like Kiss can't be trusted to play this music non-stop like we do and other stations still don't play underground stuff at all. I can't even imagine London without pirates - there'd be nothing to listen to. I think pirates keep the airwaves alive, whatever sort of music they play." By now last night is beginning to catch up with the Naturalist. He's off home to bed and Joanne's already left, but you know they will be back soon enough. Pirate radio is an obsession for them and they aren't about to be put off by the Government, the DTI, the police or anyone else. As long as there is music that isn't getting airplay on legal stations, independents like Déjà vu will continue to give it an outlet in the only way they can, illegally, on pirates. And while the DTI might not appreciate there efforts, there are thousands of clubbers and music-lovers who do.