On the night of 10 December 1968, while we were staying in a rented house in Praia da Luz in the Algarve, we were wakened by a tremendous banging on the outside gate. 'Wake up, Hunter Davies, you bugger.' Someone was shouting in a strong Liverpool accent. I thought at first it was John, it was so raucous. I got up, went across the garden, opened the gates and there was Paul and a blonde I had never seen before, plus a young girl of about five. A taxi driver was waiting for his fare, so I paid him.

That evening in London, Paul had decided he would like a holiday with us. Someone was detailed to book plane tickets, but there was no plane. Paul, like all the Beatles, couldn't wait, wanting it now, so a private jet was hired, for Paul's pleasure. The decision had been so impulsive that they arrived with no foreign money. At Faro airport, which opened only the previous year and was still pretty primitive, Paul thrust pounds 50 into someone's hands, asking for escudos, then forgot about it, jumping into a taxi and waving our address.

Paul had a bottle of whisky for me, how kind, and introduced us to Linda. When we'd left England some nine months earlier, Jane Asher was the girl in Paul's life. They'd been for tea and we'd got on well. Who was Linda? A one-night stand, or what?

They stayed eight days. Linda was naturally wary of us. We had met Jane, so perhaps she expected we'd be critical of her. She also wanted to get Paul to herself, as it seemed to be the first stages in their romance, while Paul was keen on long, late-night chats and philosophical discussions, especially with my wife. She's good at all that stuff. It seemed to us that Linda was a 'yes' girl, who was overdoing her adoration, clinging on to him, hanging on every word. We couldn't see it lasting. We couldn't see what she was giving Paul. At times during those eight days there were some frosty moments.

Paul was brilliant with the children, all of them, and others who came to the house, though we did have one or two disagreements. I went mad when our son Jake, aged two, started playing with a carving knife, but Paul said they should discover danger for themselves. He let our daughter Caitlin, aged four, drive the car they hired, sitting on his knee, which I said was stupid. Just wait till you have kids, I said.

Paul brought his guitar and messed about on it most days, even going to the lavatory with it. He started a novel but would never let us see it.

One night we talked late about marriage, marriage in general we presumed, not necessarily with Linda. When he got married, he said, it would be a public statement, a declaration about his wife: 'You I will never put down.' My wife said this seemed rather negative, why couldn't he make it a positive statement?

Three months later, back in London, they got married on 12 March 1969. Linda was three months' pregnant at the time with their first child, Mary. (Was she conceived in our little house? Could have been, judging by the dates.)

It wasn't just us. Millions of Beatles fans around the world thought it wouldn't last. Jane Asher had more or less been accepted, as she was British, one of us, had her own career and was clearly not a groupie. The world was pretty horrid to Linda. American, divorced, one year older, a photographer who had hung about with many other pop stars. Then when she turned up playing in Wings, his new group, when she obviously had little musical talent - gawd, the knives were out.

I always admired him for forming Wings, knowing he would be rubbished. When you've been a Beatle, where else do you go? Out of your mind, is one answer. Total self-indulgence is another. It takes some maturity to sit down and think: now, what I've always liked doing best in the world is singing on stage. As I can't go forward, why don't I go backwards? So he went on the road again, doing college gigs.

I also admire him for putting his family first. He had Linda play with him - which was never her desire, despite what people said at the time - so she would always be with him, and brought the kids along whenever possible. I can well believe that in 25 years they have had only 10 days apart (a little matter of a Japanese jail, accused of possessing cannabis).

We were all so wrong about the marriage. Some Beatles commentators have maintained that Linda gave Paul confidence and self-assurance, that's why it worked. I never thought he was lacking in such qualities. (Except perhaps in relation to John, but that's a study in itself.)

Linda has turned out stronger, wiser, sharper than we'd first imagined. In the early Beatles, George was always thought to be the boy, carried along by the men, but he grew with age, becoming an equal partner and at times a leader. Linda has done the same, leading Paul into new fields, new values.

Nobody can ever know, from outside, why a marriage works. They've had their rows, arguments over mainly trivial things, but there has been no hint of scandal. His children have turned out natural, well-adjusted - as much as they can be, with one of the richest men on the planet for a dad.

Paul is conservative, steady, sensible. His parents had a good marriage (despite his mother's tragic death when Paul was 14), and Paul greatly admired both of them. He waited until he knew he was ready for marriage. He was almost 27 at the time, not old by today's standards, but you have to remember he'd had 10 years of sexual experiences. Since then, he has sealed himself inside in his family life, avoiding clubs and parties and ever-eager young women. A subconscious or a deliberate attempt to avoid temptation? Or just age? Who knows?

He is still ridiculed for his tiresome assertion that he is an ordinary bloke, and he should certainly stop all that silly thumbs-up stuff, but inside the security fences, behind the fame and wealth and the posed photographs, it is very much an ordinary marriage. As with most ordinary marriages, the secret is simple. It's based on love. And they made it work - because they wanted to make it work.

Happy silver wedding.

Hunter Davies's updated version of his authorised biography, 'The Beatles', is published by Arrow, pounds 9.99.