"It was the sound of London when I was doing my GCSEs," says 31-year-old Mike Finch. "I remember in 1994 and '95 you would walk down Oxford Street and the stalls there would all have jungle playing; that Congo Natty ragga sound. It was there in the air anyway, and all you needed was a little radio to tune in." In many ways, Finch is not unusual – a rave everyman who had his life changed for ever by the music of the pirate radio stations. What makes him different is that the legacy he carries with him is also a tangible one: Finch has what must be Britain's largest archive of 90s pirate radio tapes. It's a combination of thousands of home-recorded C90 cassettes, and hundreds of "tape packs", the semi-official collections of recordings of major jungle, drum'n'bass, happy hardcore, garage and grime raves. It's music that scarcely exists in any industry-recognised recorded form.

Now a new documentary by film-maker Rollo Jackson, Finch's old friend and raving buddy, charts this remarkable obsession. Tape Crackers makes captivating viewing to which a mere description can't do justice: for 80 minutes, Jackson interviews his friend as he goes through his favourite tapes, talking us through the different stations, sub-genres, DJs and MCs, even his favourite brand of TDK cassettes – grinning broadly throughout. His enthusiasm is infectious, and in person he is no different. These days he's a father and has a job that demands he wear a suit, but as we spend an hour in a north London beer garden chewing over the British rave music of the 90s and early 2000s, there's no sad-eyed nostalgia or desire to claw back lost youth – just a beaming recognition of how "completely brilliant" it all was.

Growing up "conveniently close to Hackney", Finch had been into bands such as Nirvana and Pearl Jam before the jungle obsession of the older boys at his school converted him. His school common room was a pan-London meeting point of these local stations with their low-radius transmitters: kids from south London would bring in their Don FM tapes, and he'd bring in tapes of north London stations such as Kool FM or Rude FM. "I remember scanning the airwaves with the dial, and they were all jungle stations – this would have been around 1995. But then I stumbled upon a different sound, and it was Dream FM and happy hardcore, with DJs like Fiaz and Swifflee, and not only was the music happier, but so were these guys, too. I remember coming into school the next day, just so excited with this music, with this sound." Where was Dream based exactly? "I think south London? I don't even know – you can't Google this stuff." He sounds pleased with this fact.

In his unwavering dedication to staying poised over the record button, Finch was a latter-day version of folklorists such as Alan Lomax, who travelled the globe throughout the 20th century recording folk songs and oral traditions on the verge of extinction. Pirate radio was not just a peripheral outlet for styles such as jungle, it was the medium: a two-hour DJ set comprising records that would rarely see a commercial release, accompanied by one or more MCs practising rehearsed lyrics that would hardly ever be nailed down into song form, or just freestyling and bantering with the DJ and listeners. No two shows were alike – so for fans like Finch, every show needed recording. Compared with the permanence and ubiquity of music in the MP3 age, the transience of pirate radio music a decade ago seems extraordinary. "The music was so fast-moving, and pirate radio was really the only way to stay in touch," Finch says. Of course there was vinyl, but this would only comprise a small proportion of the tracks aired on radio. "And if these records ever did come out properly, it would be fully a year or two later."

One of the most poignant scenes in Tape Crackers sees Finch cue up a recording of one of his favourite unreleased happy hardcore tunes ("straight away I'm getting goose bumps"). He proceeds to tell the camera he never heard it again in a club or on the radio, never found out what it was called, who made it, or which DJ was playing it. There's a lot of romance to pirate radio, as anyone who's ever waltzed across their bedroom floor with a radio aerial trying to get a clear signal can testify. "It mattered a lot. I was lucky to have good reception normally, but those times when the signal wasn't clear enough, it was always painfully frustrating, when you could hear a great set going on under the fuzz. Stations would get raided by what was then the Department of Trade and Industry, or would take down their transmitters, and it was always a relief when they came back."

With no Rajar listening figures to measure their audience, the patter of the pirates is always full of phrases such as "give us a signal", "let us know you're locked" – asking for shout-outs via text message (or pager, originally) to indicate they're not just playing this music into the void. "I always felt I was one of many thousands that would be listening," says Finch, "doing my homework with it on in the background. And that's a feeling of inclusion and connection with all these people you don't know. But you'd go to a rave that weekend, and just know that some of them, a lot of them, have been listening during the week, with you."

Whether jungle, garage or happy hardcore, these were dance genres, and the essential context was the club: the raves and the radio go hand-in-hand for Finch. "You can't have one without the other. You could only go to the raves once a week at best, really, so pirate radio filled up the rest of the week." The raves he started attending with Jackson while at school were often connected to his favourite stations. "The first rave we ever went to was a Dream FM all-nighter at Adrenaline Village in Battersea. Adrenaline Village! What a name. We would have been about 16 – that was just brilliant. It's not too much of an overstatement to call it life-changing, because it just opened up a whole world, which I ultimately immersed myself in for 10 years.

"Having the rave tape-packs, and the flyers – all the memorabilia – are great touch-points to look back at now. Something tangible from all that experience." He cracks up laughing. "Because of course that experience, at the time, is a blurred affair. There are moments in a rave when the perfect combination of a DJ and an MC are just totally brilliant – so it's nice to listen back on the tape-pack when you're sober and confirm that yes, it genuinely was."

While Finch's dedication to his collection is unquestionable, it's almost too vast to be archived properly. There are, he reckons, about 150 commercially produced tape-packs (each containing six or so cassettes) – but what about his own pirate radio recordings? "I've never tried adding them up. I would think there'd be maybe as many as … 2,000? I have them in boxes, bin liners, I have holdalls, and I've still got an outpost at my parents' house in a big chest. I've got so many unmarked tapes – I didn't label them all, and to go back and do them all is just an overwhelming task. I used to wonder if there was someone I could employ to do that."

Finch has the insatiable appetite of the collector, desperate not to miss anything. At one point he was considering writing to the Department of Trade and Industry: "I don't really believe it any more, but I used to think that as part of their monitoring of pirate radio, they must be recording all of them, for any telltale clues as to where the studios might be, with rooms of government officials locked in, listening intently." He laughs. "And I thought I might be able to write to them and they'd give me access to this mythical library."

The collection is at least fairly secure from decomposition or damage, for now. Andy Linehan, the curator of popular music at the British Library sound archive, assures me tapes are a "reasonably durable medium". He says the library would be interested in a hypothetical project to encode Finch's collection for posterity, for the edification and education of the raving nation at large. "And how many tapes does he have?" Around 2,000. "Ah. Right. That might take a while." But he's in complete agreement about the principle of saving this music: "[Finch] is preserving a culture in its developing form – and that's why it's important."

One such archiving project already exists. Grimetapes was launched by Paul Lynch to collect and encode to MP3 classic grime pirate radio sets from 2002 to 2007. It is a treasure trove of lost and emerging talent, available to download for free. Many sets feature MCs who now have Calvin Harris-assisted No 1 singles under their belts, or DJs who now ply their trade on BBC 1Xtra or Kiss. One legendary 2002 tape of DJ Slimzee with Dizzee Rascal and Wiley was selected by Fact magazine as one of its albums of the decade.

It may seem premature to be looking to save pirate-radio tapes for the ages, to preserve them behind glass, but times move fast. Only a fortnight ago, the Oxford English Dictionary announced that while "sexting", "jeggings" and "woot" were being admitted to the dictionary for the first time, "tape cassette" was being removed, considered too outmoded. Just don't tell Mike Finch.

Tape Crackers is out now at thetrilogytapes.com/ttt. Grimetapes can be found at bit.ly/grimetapes