It is the opening of Tate Modern and Tony Blair is in his cool Britannia heyday, feeling pleased with himself for correctly identifying a pile of bricks as an exhibit. He seizes the hand of the nearest scruff and tells him, with that customary stare of sincerity, how much he admires his art. Which would be mighty flattering, except his new best friend is actually a neurosurgeon.

Even allowing for Ian McEwan’s magical powers of imagination, the scene from his new novel, Saturday, seems too vivid to be pure fiction. “Yes, it was actually me meeting Blair,” the Booker prize-winning author smiles sheepishly. “I just changed it from being a writer to a brain surgeon. Nick Serota, the Tate’s director, introduced us, saying, ‘This is Ian McEwan’, and Blair took my hand in that political way and said, ‘I really love your work. I’ve got two of your paintings’.”

McEwan corrected him, but Blair still insisted that he had McEwans on his walls: proof that nobody does sincerity as convincingly as Blair, because his lies deceive even himself. In Saturday, set on the day of London’s anti-Iraq war march, McEwan uses the insight to weigh up the prime minister’s truthfulness; the neurosurgeon watches Blair on television spelling out his case for war. There is no discernible doubt.

McEwan used to be as miserable and as macabre as he was right-on and left-wing. But at 56 he has suddenly turned warm and cheerful.

His hero in Saturday is happily married and even enjoys sex — with his wife. This might reflect a change in McEwan’s life. He endured a horrible divorce a decade ago (Penny Allen, his former wife, went public on her marriage hell) and an even worse custody battle after she defied a court order and took one of their sons to France.

Later he was stricken with grief for his mother, who died after a long slide into dementia. Now he is remarried, re-energised and rejoicing in even greater popularity.

McEwan, Martin Amis, Julian Barnes: for a quarter of a century they have been the three graces of English letters. And Saturday confirms how they, particularly McEwan, tower over the competition.

McEwan thinks it might be because they had to work harder to get a start. “It used to be virtually impossible to get published. Now there are too many novels. There seem to be more authors than readers,” he says. Despite the rivalry, he insists he is still good muckers with Amis, who he thinks receives an inexplicably hostile press, and Barnes, who he says has been in hiding finishing a novel.

He and his second wife Annalena are heading out to Uruguay to stay with Amis; there should be some lively debates poolside. For while Blair comes out of the book badly, anti-war protesters fare even worse.

McEwan admits he has found himself “at odds” with friends in the liberal intelligentsia “with whom I’d naturally expect to agree”. Indeed, his donnish uniform — blue cords, woolly jumper — would point to a different set of opinions but, as he says, this war is unusual.

He was appalled by articles cheering on the insurgents. “I would have thought that was a vote for anarchy and probably another one-party state.”

He loathed the anti-war slogan Not In My Name. “Its cloying self-importance suggests a bright new world of protest, with the fussy consumer of shampoos and soft drinks demanding to feel good, or nice.” Walking past marchers rather than with them, he says: “I was troubled by the sheer level of happiness on the street. I did think whatever the reasoning of America for going in, history has offered us this chance to get rid of Saddam. If you decide you don’t want that, it is probably a very reasonable view, but it is a vote for more torture, more genocide. It’s a sombre, grave choice.”

He began as an opponent of war, having “megalomaniac, insomniac” fantasies of getting to Blair and managing to talk him out of it. “There were anxieties Baghdad would be razed, the UN estimated there would be 3m million refugees and half a million dead, although we might get there yet. I did feel there was a humanitarian argument to be made and was very disappointed the government never made it.”