Imagine if John Lennon was 65.

What would John be doing now, if he were still with us today? Celebrating his 65th birthday, that's the easy answer. Whatever his circumstances, I'd like to think he'd give a party similar to the one in 1967 for Magical Mystery Tour when all the Beatles' friends and family were invited. It was fancy dress and John turned up as a greasy rocker with drainpipes, drape jacket, hair in a DA and brothel creepers - just as he used to be 10 years earlier.

He had such a good time, prancing about, posing and preening. (I went as a Boy Scout - how pathetic). It was the style and image of himself John liked to project, the hard man, "who you lookin' at, see you outside", which of course was laughable.

He'd run a mile at the first hint of trouble. The difficult answer comes in two parts. Let's say he'd never become a Beatle, just gone through life as - well, whatever life had thrown at him. which on all the evidence of his known talents and qualifications appeared unlikely to lead to very much.

He got no certificates at school, not even O-level art, much to his fury, and never passed any of his art college exams. Hard to imagine him getting any sort of job, however humble, in the art or design world. Apart from anything else, he was extremely untidy and messy, not that he was bothered. In art classes, the ones he always despised most were "the neat fuckers".

If not art, music? Again, he was never, on his own, looked upon as anything special as a guitarist, nor even as a singer. He was hardly any better than the five other original members of his first group, the Quarrymen - none of whom ended up as musicians. (One went to Cambridge and now works part-time as a lecturer in tourism, one became an upholsterer, another a care worker, one opened a little chain of dry cleaners, the other became a millionaire after building up a chain of steak houses.)

The arrival of Paul was the vital spark, igniting something that had not previously been apparent. But without the miracle of the Beatles, what would John have done? Paul, no question, would and could have made a living as a musician, being by far the more naturally talented. Music flowed from Paul. With John, it needed forcing out of him.

Failing a career as a musician, Paul would have gone to college, stuck in and passed his exams, and become a teacher. (Which, in a way, he has, the Mr Chips of popular music.) John, from all we know of his character at that time and later, would not have stuck at anything. He would have bummed about, fallen out with all his employers, chopped and changed, becoming a rolling stone, small letters please.

When I was doing my biography of the Beatles in 1967, I was desperate to track down Fred Lennon, John's father. He had abandoned John when he was very little and never reappeared, until one day in 1964 he happened to read that John was now something called a Beatle, and incredibly rich.

Here's a few bob, he thought. All he got was the bum's rush, then he disappeared. I finally tracked him down to a roadside hotel at Hampton Court, not far from where John was then living in Weybridge. He was working as a dish washer. I got his version of his life story, and why he had left John, which of course was invaluable to me whether it was all true or not. He also gave me a snap of himself, taken on board ship in prison uniform. (Something to do with a missing bottle of whisky, not his fault of course). The likeness to John was eerie, giving clues to the John to come.

I told John all about Fred, his version of events, and he agreed to meet him secretly. John didn't want his Aunt Mimi, who'd brought him up, to find out. She would have been furious. John gave Fred quite a bit of money, enough for him to get new teeth, move into a flat with a 19-year-old girl, marry her, have a son. John thought it was hysterical.

"If it hadn't been for the Beatles," John told me, "I would probably have ended up like Freddie." But he didn't. He ended up rich and famous and married to Yoko. Would that be the position today?

Still rich and famous, certainly, as even John at his wildest could not have spent all the monies which have since flooded in. The further we get from the Beatles, the greater their importance and influence has become. But it's often forgotten now that at the time of John's murder in 1980, he had become a bit of a joke figure.

He'd disappeared somewhere in America, something of a recluse, with that funny, dopey woman, doing silly dopey things, taking lots of drugs, not having produced anything for years, what a shame, what a waste. His sudden, tragic death catapulted him back into our consciousness - and to the reality of what he had given us.

So, mega-rich still, awfully famous but I doubt if he'd still be with Yoko. He'd already have gone off with someone else, fixed up by Yoko, with her blessing.

Would Yoko have been so wise, compliant or just cunning again? And again and again? Unlikely. I think there would have been several more partners, probably wives, as he was daft enough, and romantic enough not to worry about the financial consequences. Like Freddie, there would have been a 19-year-old Thai bride, or similar, plus more children.

No affairs with blokes, though. Suggestions of homosexuality, which still get aired, are cobblers. It is true that after the birth of his son Julian he left his wife Cynthia and went off with Brian Epstein for a twosome holiday in Spain. Something did happen between them, so John told me. He could of course have been lying, saying it for effect. Whatever it was - possibly little more than mutual masturbation - John would have tried it once, for a laugh, to see what it was like.

Politically, if with us today, he would have been on all the anti-Iraq marches. He'd have appeared naked in the Trafalgar Square rally, burst into George Bush's bedroom and set fire to his stetsons, broken into 10 Downing Street and smashed all Tony's guitars. For peace and love of course.

He also had eccentric causes he supported. This weekend, Mr Wolfgang, who shouted "Nonsense!" at the Labour conference, would be living it up in New York at John's apartment, having been flown out on his private jet, plied with champagne, endless spliffs, whatever he requested. John would now be buying him a house in Westminster - handy for heckling Parliament.

I don't think he would have supported Live Aid. Too slick, too successful, too glib. And I bet he would have turned down a knighthood, just to taunt Paul and Mick. But he might have stood for Parliament, on the Save the Tortoise ticket. And got in.

I think he'd still be living in his vast Dakota apartment in New York. People of his age, and musical interests, grew up loving America. But he would be growing homesick.

At the height of Beatlemania, the Beatles had no interest in the Beatles, where they'd come from, how they'd got here. They were only interested in today and, even more so, tomorrow, with its promise of new sensations.

I noticed with Paul and George that it took them till their fifties to realise something unusual had happened to them when they were young, which obviously wasn't going to happen again.

They began, often secretly, using other people as front men, to acquire items about themselves. It happens to us all, with age. The past gets more interesting and somehow clearer. Before John's death, that process had begun, asking Aunt Mimi to send over his old school drawings and tie.

I think that nostalgia could have continued. By now, he'd have bought a place in Liverpool. Not out in the Wirral, like Paul but a large semi with pebble-dashed walls and stained-glass doors in Woolton, using it as a holiday home, dangling his grandchildren on his knee, taking them for walks to Strawberry Fields, telling them about his childhood. He would be nice to Cynthia, and Julian, making amends, seeing them all right.

Paul has backed a musical college in Liverpool. John would have bought and funded his own art college in Liverpool, where there would be no exams and all the students had to produce were cartoons. It would now be world famous.

Two little memories of John keep coming back. While on the book, I wrote the obituary for The Times of Brian Epstein. The paper then asked me to do all four Beatles. I told them about it later, that it was standard practice, to have in the vaults basic biogs of well-known people. They would not be used for years or, in their case, being so young, for decades and decades. They all frowned, made faces. Only John asked to read his, which of course turned out to be the first to appear.

Another time I had gone to his home, by arrangement, on a day he had then decided not to talk. We had lunch, not talking. Watched children's TV, not talking. Had a swim in his pool, each of us going round and round, in silence. As we swam we heard something from down the hillside towards Weybridge. It was the sound of a police siren giving that familiar wailing. John picked up the rhythm and played with it. Back in the house he continued humming, creating a tune out of nothing.

It didn't appear on any record, for a long time. John always had lots of half tunes which never got finished. Then, on Let it Be, I could hear it clearly in "Across the Universe".

Happy Birthday, John, wherever you are. Hope you are still picking up the tunes.

Hunter Davies's The Beatles, the only authorised biography, is available from Cassells for £15.