You do realise, I say to Ian McEwan, this is just like a McEwan novel. “People say that, but I’ve not written a novel as good as this. It’s more a Victorian novel, more Dickensian.”

McEwan is the leading British novelist of his generation. From bleak, frequently violent early works of the 1970s like First Love, Last Rites and The Cement Garden through to the dazzling historical novel Atonement, and Saturday, his meditation on the aftermath of 9/11, he has dominated British fiction. Unusually his critical success has been matched by his popularity. A new McEwan is likely to sell 200,000 copies in hardback in the UK. Most prominent literary novelists would be happy with a tenth of that figure.

One central theme of McEwan’s work is the power of a single moment to change lives utterly. Atonement pivots on a little girl’s brief, petulant act, and his new novel On Chesil Beach is about a single disastrous sexual encounter. But life imitates art. And now it is clear that McEwan’s own life was changed by a secret meeting at Reading railway station in 1942 of which he knew nothing until five years ago.

In 2002 he discovered he had a brother called David Sharp. Until then he had only known a half sister, Margy, and a half brother, Jim, also known as Roy. Both were born to his mother’s first marriage to Ernest Wort. He died in action in Belgium in 1944. Rose then married another army man, David McEwan, and on June 21 1948 Ian was born.

After Ian, Rose was unable to have more children and they considered adoption, but nothing happened. Ian thought he was his parents’ only child, though he always kept in touch with Margy and Jim. Then he discovered that war had touched his life as intimately and shockingly as anything he had imagined in his fiction.

“My feeling is,” he says, “that there are consequences that flow from the turmoil of war that never find their way into history books or even memoirs.”

For while Ernest had been away in north Africa, Rose had taken a lover, David McEwan. She had McEwan’s baby but, with her husband returning, the lovers agreed they could not keep the child. In December 1942 they placed an ad in a local paper: “Wanted, home for baby boy, age 1 month, complete surrender”. It seems, I suggest to McEwan, a terrible thing to do.

“Well, I think one has to think back to the shame that was attached to a soldier abroad on active service in the war and a baby born illegitimately to another man. The best I can say for it is that I can understand there would have been hell to pay. But it’s hard to understand from my point of view the secrecy that followed for the rest of their lives.”

What did he think went through the minds of his parents? “This was not a generation of touchy-feely fathers and my father was brought up on the tough side of Glasgow and he was a lifelong military man and he would have thought this was a tidy solution. My mother was not strong enough to resist him.”

A couple, Rose and Percy Sharp, answered the ad and the “complete surrender” of the baby took place at Reading station. They lived quite close and, on the baby’s first birthday, the Sharps received a birthday card from his true parents. But that was that; there was no further contact.

“I can hear my father rationalising it: we must not interfere with the adopted parents and the child. I know my mother would yearn to see him but she was much too timid. Now, in retrospect, I keep reinterpreting events — the fact that my mother, after I was born, was no longer able to have children and, when I was about seven, they considered adopting. Now that seems so painful, to think what kind of atonement was involved in that or the ironies in the fact that the child they’d given away was living only 20 miles away. They must have known that.”

McEwan’s parents took their secret to the edge of the grave. David McEwan died a lingering death from emphysema in 1996 and his wife fell victim to Alzhe-imer’s. But she was still alive in 2002.

David Sharp, meanwhile, knew he was adopted and had been growing increasingly curious about his real family. He hesitated but eventually contacted the Salvation Army, who tracked down Margy. He then learnt about his brother Ian and they exchanged letters.

“My first feeling was excitement — I’ve got a brother! But also shock and then, within a few days, sadness.”

They met in a “conference-centre type pub” in Oxford. “There was a kind of comedy when we first met. We shook hands and maybe even half-embraced. I asked him if he’d like a drink and he said red wine. I went to the bar and this girl couldn’t work out how to open a bottle of wine and she had to get somebody else. It took about 20 minutes to get a glass of wine. We’d met and then immediately been separated.”

McEwan saw at once that they were similar in appearance. But did he feel any basic biological sense of brotherhood? Typically — he is gripped by scientific ideas — he refers to recent research on siblings.

“Well, we share half each other’s genes. But there has been a lot of interesting work on incest among unrelated children raised

together. They very rapidly acquire an incest taboo. Everything genetic needs signalling so it’s all about being raised together. Many people loathe their siblings but still seem to be connected because of this shared past. This falls outside that. I feel close to David because I know intellectually he is my brother, but it’s not quite the same as if we had a childhood together.”

Genetics also seemed to play no part in the outcomes of their lives. Ian has been driven from the start by intellectual and artistic concerns; David hated school and has worked all his life as a bricklayer.

Their relationship now seems comfortable. Ian does not share David’s lifelong passion for Reading football club, but their talk is easy and their families regularly see each other.

Ian plainly respects David’s “forgiving nature” and the maturity with which he handled the reunion. But in the event his timing provided yet another twist to this tale. Ian took David to a nursing home in west London to see the mother who had “completely surrendered” him at Reading station. But her dementia was too advanced.

“We both provided hints just to see if we could get through. But it meant nothing. At that momentous moment when he might meet one of his parents, there she was, alive but not there. But he dealt with it very well. He was very decent and mature about it.”

The experience has left McEwan with a sense of shock and wonder at the secrecy of his parents. “We talked a great deal and often about the past. They had endless opportunities to tell me. I can only think the shame was too great.” THE story — and the way he thinks about it — will make any McEwan reader gasp at the parallels with his work. In particular, his greatest novel Atonement, published in 2001, the year before he discovered his lost brother, has the same theme of redeeming a terrible event from the past; and, even more startlingly it shows how the chaos of war transforms people’s worlds in ways the historian seldom notices.

On Chesil Beach also has some parallels, but McEwan denies his writing has yet been affected by the reunion. “I haven’t yet absorbed it into my work. But I think I will deal with it head on in a memoir.”

David, with a ghost writer, is writing a memoir entitled Complete Surrender. McEwan is encouraging him but keeping his distance. “It’s his story. I will one day write my version.”

McEwan tells me the tale freely though hesitantly as if he is struggling to turn it into one of his own stories. Yet I feel an eerie sense that somehow his fiction has retrospectively created his life. Whenever he is in the news for not strictly literary reasons, the stories always seem to have a dark, McEwanish colouring.

His divorce from his first wife Penny Allen was messy. The messiness became headlines in 1999 when Allen absconded with their youngest son Greg to France. McEwan had custody of both their boys. He is now married to the journalist Annalena McAfee. They live in the house in London, near the Post Office Tower, that he gave to Henry Perowne, the hero of Saturday.

Last December he was in the headlines again when he was accused of plagiarism. Atonement, it was said, owed everything to No Time For Romance, the autobiography of Lucilla Andrews, the novelist. In particular he had used her accounts of nursing injured soldiers after Dunkirk.

It was a peculiarly cretinous charge since McEwan had acknowledged Andrews in the book and all novelists, particularly when writing historically, must use some nonfiction sources. But the prominence given to all three stories — the brother, the divorce and the plagiarism charge — are all evidence of McEwan’s stature in the national imagination. Increasingly he is seen as our national writer.

McEwan accepts his role as a public figure, “but I do turn down 19 out of 20 invitations to do some punditry”. He is deeply involved in the debate about climate change and, as his comments on his brother demonstrated, he has a sophisticated grasp of genetics. Saturday was his fictional response to the post9/11 world, but in the immediate aftermath he wrote a moving article about the horror of the event. These are things which, he believes, are of concern to the novelist, issues to which he might have something useful to contribute. In the case of 9/11 in particular it was the novelist’s ability to project himself into the minds of others that allowed him what was, in effect, an “expert” response.

“For me the moral core of the novel is inhabiting other minds. That seems to be what novels do very well and also what morality is about: understanding that people are as real to themselves as you are to yourself, doing unto others as you would have done to yourself. I’ve taken a lot from cognitive psychology about the way children develop theories of other minds. It’s a part of the emerging into full consciousness. People who are psychotic or autistic can’t read other minds. They might be frighteningly logical but they have no emotional commitment. We are all on a scale between an icy pursuit of self-interest and being overwhelmed by an awareness of what other people think.”

The novelist imagines his way into other minds. The terrorist cannot, or refuses to do so. “Extreme cruelty,” says McEwan, “is a failure of the imagination.”

McEWAN the public figure, the literary grandee, is a long way from the McEwan who came to prominence with First Love, Last Rites in 1975. Then he wrote spare, bleak fictions that were deeply and perhaps excessively influenced by Franz Kafka and French existentialism.

“Now when I see writing like that my heart sinks. This is staring-at-the-wall fiction. Now what I want in my reading and writing is a realised world that I can judge by some kind of recognition. And character, too, that great and wonderful 19th-century invention!”

The spare bleakness has given way to a lush and loving accumulation of detail and a careful recording of what he calls the “white noise” of passing thought. He now feels able, he says, to put more of himself and his experiences and feelings into his books.

“For example, all the things I love about the English landscape I loved in the Seventies when I wrote grim, existential stories. But I just didn’t have the confidence or the clarity of something or other to include them.”

Like the great French novelist Flaubert, McEwan believes that anything becomes interesting if you look at it closely enough. Realism is all the novelist needs, not fantasy or escape from the real world. This reinforces his place as a national public figure. He is writing about us and our society. He is our mirror, reflecting and revealing our lives, loves and predicaments.

Specifically, it is our social history that forms On Chesil Beach. It is set in 1962, representing the moment at which postwar austerity and social mores were just on the edge of breaking down, to be replaced by the freewheeling carnival of the later Sixties.

The couple whose wedding night we contemplate are trapped in the old ways and inhibitions and there is an overpowering sense that, had they met and married a few years later, all would have been different, perhaps better — though, of course, there can be no certainty about this.

It is also a novel that betrays the mature McEwan’s sense of his own readership. The book is full of his characteristically well handled tension. Seasoned readers will expect some appalling moment of revelation or act of violence. But in the event the novel’s depth is created more by what doesn’t happen than what does.

“It was never going to be that kind of novel. It was about the parting of fates over something quite small.”

In spite of his denials, I find myself thinking that his experience of discovering a lost brother must have something to do with this. But ultimately that is the point. The fiction and the life are closely but indirectly linked. That is the nature of realism.

TS Eliot once said that the great writer creates the critical language in which he is to be understood. That is precisely what McEwan has achieved. But in a sense he has gone even further. For why does the story of his discovery of David carry such resonance and poignancy? Because his understanding of our world, of what we take seriously, of what ultimately matters, has infected us all.

We are what McEwan’s mirror reflects, uncertain creatures yearning for atonement.

On Chesil Beach by Ian McEwan is published by Jonathan Cape at £12.99

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