For three years, in the late 1980s, I was the keyboard player with the Frank Sidebottom Oh Blimey Big Band. Frank wore a big papier-mâché head, and the act involved doing oompah versions of pop classics. Those were his zenith years. We toured the country, playing to sell-out crowds in medium-sized venues. We rode high. And then it all went wrong.

Now, 15 years after I last spoke to Frank, I receive an email out of the blue: "There's an 8ft mosaic of the Oh Blimey Big Band going up in the Tate Gallery next month. You know it is, it really is, thank you."

"Could that really be Frank?" I think. "Frank?" I email back. "Is that you?"

In 1987, when I was 20, I put on concerts for the Polytechnic of Central London. I became friendly with an agent from Cheshire called Mike Doherty. He looked after Ronnie Corbett, but his true calling was being Frank Sidebottom's drummer.

One day Mike phoned me in a panic and said: "Frank's playing a show in London tonight and our keyboard player can't make it. Do you know any keyboard players?"

"I can play the keyboards," I replied.

"Well, you're in!" he shouted.

"But I don't know any of the songs," I said.

"Can you play C, F and G?" he asked.

"Yes," I said.

"Well, you're in!" he shouted.

I arrived at the venue, the Cricketer's in south London. Frank was preparing to sound-check. Even though the concert was a few hours away, he was already wearing the papier-mâché head: two wide bug eyes staring, a mouth frozen into a half-smile, red lips slightly parted, as if mildly surprised, very flat hair, a permanent tan - there in the dingy half-light of this empty pub. He was fiddling with some equipment. Why was he wearing the head when there was nobody to see it except the band and the pub landlord? Did he wear it all the time, like Michael Redgrave with the ventriloquist's dummy in Dead of Night? Close up, the head was bumpy. (Later, in the early 1990s, he made a far smoother fibreglass version.) I knew his real name was Chris, so I approached and said, "Hello, Chris. I'm Jon."

He ignored me.

"Hello Chris," I said again.

He continued to ignore me.

"Hello, uh, Frank?" I tried.

"Hello!" he yelled.

Mike the drummer came over to explain that when Chris puts on the head, Chris ceases to be. And sure enough, during the years that followed, I never heard him respond to the name Chris while dressed as Frank. I discovered why he was in the head hours before the show: he'd been doing an event at a record shop earlier that afternoon, and his habit - in situations like that - was just to stay being Frank.

"We'll put you over here," said Mike, setting up a keyboard stand for me at the side of the stage. Frank gave me a quick lesson. The songs were, indeed, comprised entirely of C, F and G. They were all covers - I Should Be So Lucky, Material Girl, Every Breath You Take - so I picked it up quite quickly. Then the audience arrived. And - I can say this because this was Frank's vision, and I was just a session musician, just an oily rag - it was brilliant. There was something fantastically warped about the act, which was four men assiduously emulating and fleshing out with real instruments the swing-beat chord sound of a cheap children's Casio keyboard, with a living, slightly eerie cartoon character prancing around at the front, singing in a nasal Mancunian twang, as if he had a swimming peg attached to his nose. Each song ended with the same words: "You know it is, it really is, thank you."

As we played that night, to a room packed with hundreds of euphoric people, including Midge Ure, I marvelled at the train of creative thought that had somehow led to this bizarre and unique place.

At the end of the show Frank introduced the band. "On guitar, Rick Sarko!" The audience cheered. "On drums, Mike Doherty!" The audience cheered. "On keyboards, Jon Ronson." Nobody cheered.

"What's gone wrong?" I thought. And then I realised. Concerned that I didn't know any of the songs, they had turned my keyboard right down and positioned my stand so far behind the speaker stack that, unbeknownst to me until that moment, the majority of the audience had no idea I was there.

"Why did they even bother asking me?" I thought miserably on the journey home. And then, a few weeks later, Mike called and asked if I wanted to join the band full time, and so I moved to Manchester.

In those days, the identity of the man under the head was the subject of great speculation. On many occasions, Sidebottom fans would barge into the dressing room before a show and refuse to leave until the real Frank revealed himself. They'd go around the room: "It's you, isn't it? No. You're Frank, aren't you?" On most occasions, the only person they wouldn't bother asking was the unassuming Chris, who blended into the wall.

By 1987 Frank Sidebottom had existed for three years. Chris had previously been in a Manchester punk band called the Freshies. They had one minor hit, I'm in Love With a Girl on a Manchester Virgin Megastore Checkout Desk, but that was all. Chris, at a loose end, sketched the character of Frank, recorded a terrible version of Anarchy in the UK with Casio accompaniment, and sent it around the major record labels with a covering letter that began: "Dear X, I'm thinking of getting into showbusiness. Do you have any pamphlets?"

Someone at EMI found this funny enough to invite him in. He arrived, dressed as Frank, and as he walked in the A&R man asked: "Have you been in showbusiness for long?"

Frank looked at his watch and replied: "10 seconds."

His debut EP, released on EMI, was called Frank's Firm Favourites. The comedy came from the juxtaposition between the parochialism of Frank's everyday suburban life and the grandiosity of the songs he covered. In real life, Frank's life was much like Chris's. Both lived in an unremarkable cul-de-sac in Timperley, Cheshire. Biographical details were revealed in songs like Born in Timperley (to the tune of Born in the USA):

I go shopping in Timperley

They've got loads of shops

That's where I do the shopping for my mum

Five pounds of potatoes and loads of chops

Chris invented a cast of supporting characters, who would pop up between the songs on his radio show, Radio Timperley. There was the puppet, Little Frank. There was the neighbour, Mrs Merton. Chris asked his brother-in-law's friend, the then BBC secretary Caroline Aherne, to do the voice of Mrs Merton, which is how she came to exist.

Paying close attention to Frank's burgeoning world was Graham Fellows, who went on to invent John Shuttleworth on Sidebottom principles, and eventually became more famous, a bit like Nirvana to Frank's Pixies. (I think Chris was, understandably, a little embittered by Mrs Merton and John Shuttleworth's subsequent successes.)

Frank's Firm Favourites charted at about number 90. EMI dropped him. But by then his following was sufficiently large to pay me £40 a night to be his keyboard player. And for two happy years, I became his booking agent, too.

Frank supported Bros at Wembley, and Gary Glitter at a Freshers' Ball somewhere. Glitter's roadies were extremely rude, cornering the band and issuing a list of do's and don'ts: "You aren't allowed to use our lights. Stay away from our hydraulic stage." And so on.

Under the head, Chris was seething. As soon as Frank went on, he jumped on to the hydraulic floor, which set off smoke bombs and rose dramatically above the heads of the audience. "Come on! Come on!" sang Frank. "Do you want to be in my gang?" Within 20 seconds he spotted Gary Glitter's roadies pushing their way through the audience towards him. He jumped off stage and ran down the corridor, pulling off his head and costume as he went - he had his own clothes on underneath - just as the roadies caught up with him.

"Did you see Frank Sidebottom?" they asked him.

"He went that way," said Chris.

We were riding high. And then it all went wrong. Chris announced he was going to make the band more professional-sounding. He hired a proper saxophone player, and an overly suave bass player called Richard. Richard played in an effeminate Manchester guitar band, but there was nothing effeminate about him. He took an instant dislike to me and, two rehearsals in, threatened to take me outside and "break" my "keyboard playing fingers".

Suddenly, we were rehearsing a lot more than we used to. We were building virtuoso sax and guitar solos into Timperley Sunset and Anarchy in Timperley. I don't want to sound wise after the fact, but it was clear to me that this was heading for disaster, and only I had the foresight to realise it.

"The audience don't want a note-perfect Oh Blimey Big Band," I urgently told Mike the drummer one day during a break from rehearsals.

"Yes," Mike replied. "But we can't just spend our lives sounding bobbins. We need to move it forward."

(Bobbins was, by the way, a word popularised by Frank. It may end up being his most enduring legacy.)

"The audience isn't going to like it," I warned.

I booked Frank a huge 30-date tour. It was decided in rehearsals that I'd begin the show. I'd walk on, alone, into the spotlight, and play a powerful C. This lone note would last a minute or more, a simple but compellingly forceful C pulsating through the venue. It would ignite the audience into a frenzy of anticipation. Then the rest of the band would join me and we'd open by thrashing out a power-rock version of Born in Timperley. The audience was noticeably perplexed. Where had our beloved sound gone? What were we thinking? We sounded like John Shuttleworth being backed by Survivor.

A few days into the tour we got an extremely caustic review in the NME. Previously, they had always liked us. Now they expressed astonishment at our new musical direction. Word got around that we had lost our talent. By mid-tour, our audiences had dwindled from 500 or so to 30 if we were lucky. Chris always said that his favourite shows were the ones where everything went wrong. There were lots of those in the latter half of the tour. In Dudley, there can't have been more than 15 people in the audience. Midway through, someone produced a ball. The audience split into two teams and, ignoring us, played a game. As we came off stage that night, Chris took off the head and said: "That was the best show ever."

When the tour ended, I lost contact with Chris and Mike. I moved back to London. Fifteen years passed. Chris wound down Frank. He didn't retire him completely: there were solo shows, a greatest hits CD, but Chris found a new way to earn a living: as an animator on Pingu. And then, out of the blue, I got the email about the 8ft mosaic, and then another email with a photograph of it. It was created by the artist Mark Kennedy. It really is going up in Tate Britain, for one day only, this Friday. Frank will be there, too, performing a solo show. A Tate curator, Andrew Shaw, is a big Frank fan, which is how this strange booking came to be.

There, in the mosaic, is the Radio 2 presenter Mark Radcliffe, wearing a fez. He was the keyboard player who couldn't make the London show, providing the vacancy for me. (He later rejoined the band on accordion.) There I am in the fez next to him. There's Mrs Merton. And there's Chris Evans with a steering wheel. This represents the fact that he was briefly, before he became famous, our driver. One time we were playing in London. We pulled up on Edgware Road. Chris Evans wound down the window and said the funniest thing I've ever heard.

"Excuse me?" he said.

"Yes?" said a passer-by.

"Is this London?" he said.

There was a silence.

"Yes," said the passer-by.

"Well, where do you want this wood?" he said.

And there is Frank. Time hasn't ravaged him. He looks exactly the same.

He is planning a full comeback. My hope is that he'll ask me to play keyboards.

