This is not Spinal Tap

Neil Innes is wearing a black beret like those worn by his Beach Boys parody band Les Garçons de la Plage. That group featured very briefly in the only rock mockumentary that can be ranked alongside This Is Spinal Tap - 1978's All You Need Is Cash, the hilarious story of the Beatles-like Rutles ("the prefab four"), in which Innes starred, and for which he wrote all the extraordinarily Beatles-like music.

Innesis best known in the UK as a kind of semi-Python. Remember the irritating minstrel in Monty Python and the Holy Grail? That was him, and he played several other parts in the movie, as well as appearing in many of the other Python films, live shows and side-projects. But it is the imminent return of his fondly remembered Rutles that has drawn us together at an arts centre in London's East End.

According to Eric Idle's narration, "The Rutles story is a legend. A living legend. A legend that will live a lifetime, long after lots of other living legends have died." And now, at last, the legend lives again. The Who's singer, Roger Daltrey, had asked Innes to re-form the Rutles for a one-off charity gig, with Steve Coogan and others, for the Teenage Cancer Trust at the Royal Albert Hall on Wednesday. But things took off. "I mentioned the gig to a friend, and they said: 'Why don't you come down to the 100 Club as a warm-up?' so we're doing that the day before. But that sold out, so then they said: 'Well, you can have the day after, too.'" It didn't stop there. He is now doing a mini British tour. "The songs are fun to play, and I think everyone likes to get back in the van and realise just how miserable it can be."

The reason there is still demand for the songs is that many are, arguably, the equal of the Beatles tunes they reference, while others are beautifully crafted musical jokes that are, unusually for musical jokes, listenable to again and again. In fact, discovering the Rutles is almost like discovering a whole bunch of new Beatles songs. As if to emphasise the quality of his writing, Innes's cult classic, "How Sweet to Be an Idiot", was ripped off by the ultimate post-Beatles rockers, Oasis. He now has a percentage in their song "Whatever".

In the 1960s, Innes was the musical mainstay of the Bonzo Dog Doo-Dah Band, for whom the late Vivian Stanshall was the front man. "I was an art student at Goldsmith's. I'd heard about these extraordinary blokes who called themselves the Bonzo Dog Doo-Dah Band and who wanted a pianist, so I met Viv in this big Irish pub in New Cross. He was quite plump in those days - he had Billy Bunter check trousers and a Victorian frock coat, pince-nez glasses, carried a euphonium and wore pink rubber ears. And no one took any notice!" Stanshall couldn't play the instrument. "No, but I think anyone with that level of enthusiasm can probably get any inanimate object to obey their will."

The Bonzos quickly became popular on the London pub circuit and landed a weekly gig on the Thames Television children's show Do Not Adjust Your Set, where, as Innes puts it, they met the embryonic Pythons - the show also featured Eric Idle, Michael Palin, and Terry Jones (alongside David Jason). They also had a hit record - the Innes-penned "I'm the Urban Spaceman" which was produced by (the first Beatles connection) Paul McCartney, under the pseudonym Apollo C Vermouth.

But eventually they split. "We went to America, and when we came back we knew that we'd had enough because we'd stopped arguing." While in America, Innes rubbed shoulders with another musical giant. "We were playing a festival in Chicago, and I was having a piss. [Jimi] Hendrix walks in and, as he's standing next to me, says, 'We're doing the same thing.' I said: 'What, having a piss?' He said: 'No, making revolutionary music. Only I have to pretend I'm serious.'"

Innes then spent a few years with the poet Roger McGough, the comedian John Gorman and McCartney's brother Mike McGear in Grimms, but living-legend-dom was fast approaching. It arrived when he collaborated in 1975 with Idle on the latter's BBC2 comedy show Rutland Weekend Television, and on an album, The Rutland Weekend Songbook. One of the songs on it, "I Must Be in Love", was a Beatles pastiche for which they had filmed a spoof Help!-style movie promo. Then, just as it did with the Fab Four themselves, an appearance on American television changed everything.

"Saturday Night Live was offering George Harrison silly money to get the Beatles back together. But they still wouldn't do it. So Eric got them to show the video for "I Must Be in Love" instead. There was a huge mailbag, which meant that Lorne Michaels, the producer of Saturday Night Live, got the money to make All You Need is Cash. That's when someone asked me: 'Can you write 20 more Rutles songs by Thursday lunchtime?'"

The 72-minute film starred Innes as Ron Nasty (the "Lennon" character), Idle as Dirk McQuickly (ditto McCartney), Rikki Fataar as Stig O'Hara (Harrison) and John Halsey as Barry Womble (Starr). There were cameos by George Harrison, Mick Jagger, Paul Simon, Ronnie Wood and many others, in a "history" of the Rutles' career. It took in films like A Hard Day's Rut and Ouch!, as well as albums called Sgt Rutter's Darts Club Band and Let It Rot.

Innes's songs were spot-on. "Hold My Hand" is 1963 beat, "Good Times Roll" is "Lucy in the Sky"-ish psychedelia, and "Piggy in the Middle" is "I Am the Walrus"-period stoner nonsense. And then there's "Cheese and Onions", the best fake Lennon song ever. Just as clever is the production, which captures George Martin's style perfectly. "While I was writing the songs I didn't listen to any Beatles tunes. What I did listen to was the production. It's like painting for the ears, getting the light right."

After the Rutles, Innes did three BBC series of The Innes Book of Records (1979-81), each show featuring a handful of original songs. He then took a curious career turn, concentrating on children's television, writing for and appearing in The Raggy Dolls and Puddle Lane.

But, in the 1990s, the imminent release of the Beatles Anthology CDs of unreleased material spurred him to return to the Rutles. "I went to see George [Harrison] and said: 'What'd you think if we did the Rutles again?' and he said: 'All right, but which one of you is going to get shot?' And I said: 'Precisely. It might not be very funny!' The result was Archaeology. "Many people think Archaeology is better than the first album," says Innes. In fact, this second Rutles collection is probably a bit more Innes and a bit less Beatles. "Yes. But if you want to try and write good pop songs you couldn't do better than having the Beatles as an influence."

Rutles gigs are rare, and only he and Halsey of the original Rutles still indulge. "John is the world's funniest man. He runs a pub in Cambridge, which amuses him to a certain extent, but he's now feeling the need to run away." Idle never actually played on the recordings and Fataar is a top session-player in the States.

The Albert Hall gig is important to Innes. "Six teenagers are diagnosed with cancer every day in this country, and they get shoved into rooms either with children or older people. The Teenage Cancer Trust is building proper units for them across the country." He's less clued-up on what the Rutles set-list will be. "I've no idea. There's 30-odd songs to choose from. It's wonderfully vague. My friend, John Dowie, says to me: 'If you don't rehearse, what can possibly go wrong?'"

And are there any more "undiscovered" Rutles songs still out there? "If it made me money to do some new Rutles songs, I'd be up the drainpipe like a rat!" Really? "No. It's a labour of love, the Rutles thing, and if we can just go out and play the songs it'll be fine." And if it isn't, there's always Les Garçons de la Plage to fall back on.

Neil Innes and the Rutles play the Royal Albert Hall, London SW7 on Wednesday. They play the 100 Club, Oxford Street London W1, tonight and Thursday, and then Newcastle, Birmingham, Bristol, Southampton and Leeds (www.neilinnes.org)