'Am I weird and gruesome?'

"I had finished my last novel, Atonement," says McEwan, "which was my long, hot summer, country house novel and I was feeling empty. All the ideas I had for that book, the things that obsessed me, had gone.

"I was looking around almost like a lost soul and I said to Annalena, 'I will set my next novel in wherever I end up living' and that place turned out to be central London during a certain period in our lives. It was post 9/11 and the war was kicking off and I wanted to incorporate those things into a book, but I also knew I wanted to describe a brain operation and this is how all those things meshed together."

He also, unlike his past novels, decided to set the book firmly in the world of work. He tells me he spent two years following a neurosurgeon around so he could write as accurately as possible about Perowne's job. "It occurred to me that work never features in novels.

The lead characters never seem to do anything that would earn them any money. I wanted a job that would show that total absorption you get when you work sometimes. It's an amazing feeling. Sometimes I am writing and I feel an elation and that's what Perowne feels when he's operating on a patient, on that sacred place we know as our brains. It fascinated me."

A lot of what McEwan writes is about intrusion; the intrusion of a surgeon into the brain, of a deranged man into family life. One man's obsession with another and his intrusion into a marriage is at the core of Enduring Love, recently made into a film. But in that book the marriage does not survive.

In Saturday, love, marriage, the family are at the core of the book. They
must survive. McEwan describes Henry Perowne looking at his wife's sleeping figure, at the indent his own warm body has made in the bed.

"Ah, that's the other thing people don't write about," says McEwan. "They write about sex, about failed relationships, about unhappiness, but it's much more difficult to describe deep-seated contentment and the love of a woman you have known and loved and trusted for a long time."

He is obviously talking about McAfee. He met her just over a decade ago, after his marriage to faith healer and therapist Penny Allen had ended, when McAfee went to interview him for the Financial Times. "We have the first words we ever said to each other down on tape," he says. It must have been a relief because the fall-out from his first marriage was as intense and weird as his books.

Essentially, after 24 years of being together (on and off at the start but then followed by 13 years of marriage and the birth of the two boys), they separated. She claimed that his head had been turned by the London literati and he, well, he said nothing.

In fact, he continued to say nothing despite much provocation from Allen and her new partner, faith healer Ismay Tremain. The whole sad,
convoluted saga came to a head in 1999 when, McEwan having been awarded custody of the children, Allen failed to return Greg to his father after a holiday in France and went on the run with him.

She was quickly found and Greg was returned. In the court case, the judge described McEwan's former wife as "self-centred", and "more governed by her emotions than her intellect". McEwan was described as being the "epitome of restraint".

How strange that a man with such an active, macabre imagination could be so restrained. I imagine it comes from his father, Major David McEwan, a man who not only affected young Ian McEwan's life but also his novels.

"Yes," says McEwan rather distractedly. "I was never very nice about fathers really, was I?" His own relationship with his father was contorted. "He was a fascinating man in many ways," says McEwan. "He was from Glasgow and really rather intelligent but he never got a chance to pursue his intellect. Instead he went into the Army and became fierce and angry."

McEwan was spared a lot of his father's wrath as he was sent away, at the age of 11, to Woolverstone Hall, a boarding school in Suffolk. "I didn't really see my father in the holidays," he says. "I just remember being terribly bored. I had to spend my holidays in German Army bases with no one to hang out with.

"I think that sense of adolescent boredom is in many of my novels. Parents, in those days, certainly wouldn't speak to you. They never played with you. They just sort of fed you and monitored you so it was a pretty lonely place to be. I talk to my sons all the time. We have a fantastic relationship."

But when McEwan was older and at university, his mother, Rose, finally told him about how the relationship between herself and his father had been. "I found out that my father drank and when he did, he would hit her," says McEwan.

"I was very angry. My parents had bought me my first typewriter and I typed a letter to him saying that he must stop his behaviour as it was unforgivable. My mother destroyed the letter. It made her nervous, she said, to have it in the house."

His father died in 1996. Did McEwan ever discuss this with him? "No," says McEwan simply. "His mind was going and all he wanted to do was to tell me about Dunkirk. Here he was, a man who was used to being looked after by his mother and then, essentially, by the Army, on his own and having to survive ... He told me this story again and again and I used it in Atonement."

His mother, who died last year, is immortalised as Lily, Perowne's mother, in Saturday. She has lost her mind and returned to a childlike state. "My mother's brain went, so nothing much was discussed with her either," he says. "I now know that there's no such thing as resolution. Maybe if I believed in God there would be, but I believe that we have this one marvellous life here on earth. Why do we need to believe in this higher power?"

Perhaps this was at the crux of the failure of his first marriage; on one side, Allen, spiritual New Age-ist; on the other, McEwan, agnostic, atheist even. How on earth does that mix? "It doesn't, didn't," he says shortly.

However, Allen's barb that McEwan is obsessed with the literati/glitterati is something McEwan does not concede, despite the fact that he and McAfee are an artsy power couple. "The worst thing anyone can have is status anxiety," he says.

"I've seen it destroy so many people. I always hope my novels will be successful, but I have to make sure I'm as happy with them as I can possibly be and then let everything else take its course."

But now, before the inevitable barrage of nominations and shortlists that will happen for Saturday, McEwan is back at the empty-vessel stage. What's next? "Actually," he says, getting excited, "I've got rather fixated with climate change. I think it will be the most important thing in our lives over the next few decades. I am going to the North Pole with a group of scientists to see, first hand, what's going on."

He suddenly says he must go. "I have a life I need to get on with," he says. A happy, contented life, here on earth? "Exactly," he says.

Saturday (Jonathan Cape) is published on 7 February at £17.99