Our story begins not in the East End - with a couple of geezers havin' a laugh and titter and an oily rag down the rubadub nuclear sub - but at Sun studios in Memphis, where Jerry Lee Lewis and Johnny Cash made rock'n'roll history, and where renowned engineer Jack Clement worked. In a letter to the label boss who introduced him to Chas & Dave's World of Chas & Dave in 2000, Clement says: "I can only imagine how much fun Chas and Dave would have been to have around in the Sun Records studio in Memphis in the rockin' 50s. I think all of us, including Jerry Lee Lewis, would be better for it and I'm pretty sure that Elvis would have been entranced by them."

Cut to this year's Glastonbury - where Chas & Dave pulled a crowd of around 18,000 to the acoustic stage. "The amazing thing was, they knew every word of every song," says Dave Peacock as he takes out his guitar. Chas Hodges, meanwhile, is setting up his new mini Fender amp. It makes him look like a giant.

Going off to get chairs, I bump into Jamie Cullum, rehearsing downstairs in the same studio. "Chas & Dave? Nice one. They were heroes of mine," he says. The Libertines are also big fans ("Pete said we made him want to become a musician," says Chas); Roots Manuva namechecks them.

Chas & Dave love the fresh wave of attention - but it's not fooling them. "Chas said a good thing to me the other week," says Dave. "We've never been right in, so we can't go right out. We don't fit in anywhere, so we fit in everywhere. Summing like that."

I've got no idea what I'm going to do on their tune Rabbit - but I insist we try it. We all shamble around a bit on our instruments, testing levels, then, as if some telepathic gun had gone off, they start, perfectly together. Dave beams at me while he is playing; it's amazing what an effect that can have on the quality of the sound. I try and beam back, but it's tough when you're playing the trombone. "Rabbit, parp, rabbit, parp, rabbit-rabbit, parp-parp ..."

And they can play - which somehow seems to shock people who assume "light-hearted" is an excuse for being crap at your instrument. "When we were writing songs, years ago," says Dave, "we were doing sessions and things - just to pay the rent. We did an album with Oliver Nelson, a New York trumpet player. It was half of Count Basie's band and half of Duke Ellington's band."

"We didn't know who they were," says Chas. "That was for a [record producer] called Bob Thiele - turned out he got Buddy Holly going. He came over with his wife, Teresa Brewer, and said, 'I want you two to do an album.' We used to use this rhyming slang, like, 'You got an oily, Dave?' And he says , 'Whassis oily, oily raaags?' I said, 'It's our word for fags.' So he called us Oily Rags on the album."

We play a few numbers with Chas & Dave on guitars - the way they like to play the first half of their gigs. I ask Chas if he'll have a go on the piano. He half dismantles it, reveals its guts, winds up his left hand and kicks off a bit of 12th Street Rag - a showcase for his boogie. "Take it away, Pascal!" I parp it away.

Chas laughs: "You got some nice syncopation there. First time we done that with a trombone ... I think." Chas's mum was a piano player, and wanted him to take it up, but he preferred guitar. "Then, later, I was in a band called the Outlaws, playing bass guitar. We went on tour with Jerry Lee Lewis, and he showed me a few things on the piano. I thought: hang on a minute, that's a different way of playing. I started copying his records, and ended up recording with him in 1972."

"Funny enough, talking about Jerry Lee, his sister's just recorded one of our songs: Wonder in Whose Arms You Are Tonight," says Dave.

"Great, isn't it?" says Chas. "All goes round in circles."

Their career is full of interesting little circles. For example, the hook used on Eminem's My Name Is was from a Labi Siffre tune, I Got The - featuring music from Chas & Dave. Then there's the whole Beatles thing. "I was playing bass for Cliff Bennett and the Rebel Rousers," says Chas. "We were supporting the Beatles, their very last European tour. You could see why they were packing up, because as soon as they went on stage, all you heard was 'Aaaaaaaaaaaa!' from start to finish. I could picture John Lennon just going, What the fuck's going on? 'She loves you yeah, yeah, aaaaaaaaaaaa!' Nobody listening. Drive you bloody barmy wouldn't it?

"Paul [McCartney] - he's a big Chas & Dave fan now, which is great, but I remember when I first started talking to him, and he played us Revolver (though it didn't have a title at the time). When he got to Yellow Submarine he looked me in the eyes all the way through it, looking for a reaction. I didn't know whether I was supposed to laugh or not!

"We started what we were doin', what we are doing now, because we were fed up with the music in the 1970s. We loved rock'n'roll, but we wanted to incorporate our rock'n'roll feeling, our own accents, and write about things that we know about. Ian Dury did it his way, Madness theirs. We termed it Rockney."

We play again - they're generous musicians, and the glow they give off when they play is infectious. Dave is still beaming. There's no one quite like them, but there is a Chas'n'Dave lookalike band, which Bill Bailey referred to on Never Mind the Buzzcocks, having seen a poster for them with a sticker saying: "This is not the real Dave." As in, not the real fake Dave? "I can't go nowhere without someone saying, 'Are you the real Dave?'" says the real Dave. Which reminds him ...

"Geezer just come back from Greece, where some bar say they've got 'Chas'n'Dave playing live, here tonight'. My missus is getting on the case. Anyway, out of the blue, I got this geezer painting the house, and his mate's just gone to the same resort. So I phoned him up and said go in that fackin' place and tell the man that's running that club: that ain't the real Chas'n'Dave!"

"We better find out, perhaps we're booked there and we don't know it," says Chas, admiring his new mini amplifier.

"Yeah," says Dave. "Perhaps they're earning more money than we are. How about that for sauce?"

· Chas and Dave's Greatest Hits is out on November 25 on EMI. They play Shepherd's Bush Empire, London W6, on December 15