You can rely on a wife to be either distracted or unimpressed at the defining moment in life, especially if she is cooking dinner for six. Gill Hornby was harassed in the kitchen when the phone rang for her husband, Robert Harris.

"Some foreigner just called for you," she said. Harris, whose agent had prepared him for the news that another of his bestselling novels was heading for the big screen, told her in his mild way that it might have been the film-maker, Roman Polanski.

"Oh, I thought it was someone from a call centre," she replied.

That was the second week of January. Two frantic months later, and after several lunch-laden meetings with Polanski, Harris had delivered the screenplay for Pompeii, based on his novel about the cataclysmic 72 hours of the eruption of Vesuvius in AD79. Shooting in Spain will begin in August - there are already 25 people working on it full-time - and the Italian rights have just been sold for $15 million (£7.9 million). It will be the most expensive film ever made in Europe.

"Polanski moved with astonishing speed," says Harris, who did the same, putting aside the contemporary political novel he is writing to take what he calls "a masterclass" in how to do a screenplay.

"There's nothing better than working with someone who's very clever. It's like having an extra charge to your brain."

It's the perfect partnership, not just Polanski and Pompeii - a natural techno-thriller absolutely tailor-made for his dark, voyeuristic skills and love of the grotesque - but Polanski and Harris, both men who thrive on the imperatives of a cracking story.

"That business of: What will he do next? What happens next? That's everything for me," says Harris, who simply can't wait to be on set, watching a fleet of triremes being ordered into the Bay of Naples under a rain of pumice as the volcano explodes in a fury of special effects and boiling waves of red-hot vapour scorch the terrified populace.

Pompeii was the book that enabled Harris, who seems incapable of writing anything but intelligent bestsellers, to break with journalism, though old habits die hard.

"I'm a journalist at heart and I believe in deadlines. Adrenaline kicks in and you are better than if you amble on. If deadlines were good enough for Dickens, they're good enough for me."

His fictional reworkings of history have sold 10 million copies in 34 languages. Fatherland, set in Berlin and based on the premise that Hitler won the war, changed his life and is still the biggest seller, but Pompeii is close behind, and no doubt the movie will put it ahead.

He picked up the idea from a Daily Telegraph news report that cast new light on the last hours of Pompeii.

"Like most people, I had thought the eruption was a big bang, but there were several days of warning: the water became poisoned, wells dried up, there were tremors. In the first 12 hours, the town just filled up with stones. All the people who died in Pompeii were found at roof level and they had been hit by a searing hot cloud that came down off the mountain when they thought the worst was over."

To Harris, history is a powerful, living thing. The past is always ambushing him with its uncanny precepts for the way we live now. That's why he was such a feared and revered political columnist: he took the wise, long view that politicians themselves prefer to ignore and there was a ring of even-handed truth about his observations that they found disconcerting.

When he went to Pompeii to test his hunch about a novel, it was a late summer afternoon and the crowds were diminishing. As he walked along the disinterred cobbled streets in the heat and "smelt water drying on stone" - who else would smell water drying on stone? - he knew he was experiencing exactly the same sensations a man would have had 2,000 years ago. Seeing the majestic aqueduct at Cumae, he almost wept.

Soon he became "an aqueduct bore and a hydraulic cement bore" as he mastered the technology of the Roman water supply system - the better to understand the problems of his central character, Attilius. You can well imagine Russell Crowe or Bob Hoskins in the role of the blunt water engineer whose discoveries and heroism are at the heart of the catastrophe.

Harris's father was a printer from Nottinghamshire, a self-taught artisan, and Harris has great respect for people with practical skills.

"They are the ones whose voices are lost to history. All we hear about are the politicians, the artists, the soldiers... but the engineers, the practical people, they really changed the world."

Harris has just turned 50, a tall man with an easy, thoughtful manner and no delusions of celebrity. His children tell him with fearsome logic that he cannot be middle-aged because he won't live to be 100. His epiphany about novel-writing came one wet Monday in October when writing Pompeii was like breaking rocks, yet he knew he'd far rather be grappling with the Roman water supply system or vulcanology than writing a 1,000-word newspaper column.

"There is nothing like the ecstasy of relief when a book is finished," he says. "I go slightly mad; buy things. I tell Gill I'm going out to buy a book and I come back with a car." (He likes them low, fast and expensive.)

They and their four children live in a tall red-brick Gothic vicarage in Berkshire known as "the house that Hitler built" because it was bought with the proceeds from Fatherland. It looks like a mini-St Pancras without the architectural icing and Harris resembles the solid Victorian entrepreneur who might have commissioned it. The lawns slope peacefully down to the Kennet and Avon canal, where men are cleaning their barges in the spring sunshine.

He thinks he is fortunate, and he is, but it is the good fortune of the self-made, self-motivated man who is very good at what he does. Being wealthy doesn't appear to have made him complacent.

"Nothing ever satisfies you. I have never written anything I think is perfect. To be a writer is a testament to optimism. You constantly hope the next one will be better."

As a novelist, he is completely his own man.

"All my life I have loathed selling my time. I am happy to work hard but I hate that feeling of clocking in and clocking out. I would go so far as to say my whole life has been a flight from that. My father had to do it and he hated it. I wasn't going to do the same."

Physically, he has been around while the children were growing up, but admits: "So much of your life takes place in your head. Often you are present but you are not really there."

Now the script for Pompeii the movie is written, Harris is eager to return to The Ghost, a contemporary novel that is expected to draw sensationally on his inside knowledge of politics. One of his great friends is Peter Mandelson and he had privileged access to Tony Blair on the 1997 election campaign trail. The "ghost" of the title is the professional ghostwriter of a prime minister coming to the end of his time in power, and though it isn't about Blair, Harris does not deny that there are parallels.

At one time he seemed a Blair enthusiast, calling him the most gifted communicator he had come across. Ten years on, is he disillusioned by Labour?

"I was never massively illusioned to start with - much to their irritation. I never wrote that Tony Blair was a genius and a wonderful thing. The only senior politician I can claim to have known really well is Peter Mandelson and the way he was treated by his friends [he was twice forced to resign] did disillusion me."

"I thought they were a pretty ruthless and unpleasant bunch. Blair has not made a bad fist out of a lot of aspects of being prime minister, except in foreign policy - but that is so colossal it blots out everything else. It's like saying Neville Chamberlain was a good prime minister - which he was - but what does it count against Munich?

"Iraq hangs over him and the wrong turning the world has taken, thanks to him and Bush, is something that will haunt the world for years to come. Still, it does seem bizarre that they should be getting rid of him and putting a less popular person in his place."

Harris is as much a loss to political reporting as he thinks Mandelson is to politics.

"How on earth could we pursue a war with Iran?" he asks.

"We haven't got the ships and we haven't got the planes. Our men in southern Iraq are incredibly vulnerable, that's the great untold story. They could be cut off by the Iranians in about 36 hours and their only hope would be the Americans. There is a great deal of posturing."

The cash for honours debacle hardly bothers him because he seems to believe in a quantum theory of humanity.

"At any moment there are a given per cent of idiots and criminals - and it will never alter. I bet the proportions never shift. I really don't believe in the perfectibility of man."

If there is real disillusion, it's with his old trade. Where, he asks, was the anger when a recent report revealed that three-quarters of a million Iraqis may have died as a result of the war? Where was the publicity?

"This is more people than were killed by Saddam Hussein. At a time when you need more information, you get less."

He finds publishing equally disappointing, with ghosted celebrity memoirs clogging up the bestseller lists and books being spun from a drawerful of ideas after an author is dead. Even fiction is ghosted.

"I could name you three bestselling novelists who have ghosts. One claims to have first made the acquaintance of 'his' book on receiving it. Yet publishers look at you with astonishment if you suggest it is morally dubious."

Harris is reluctant to talk about a book in progress - he is a superstitious man - except to say that The Ghost provides a welcome holiday from ancient Rome. (His last book, Imperium, about the orator Cicero, is intended as the first part of a trilogy.)

"There is a certain satisfaction in writing about characters who don't lie down when they eat and wear togas and have complicated names. It is quite nice to get back to contemporary times."

You seldom meet a man as happy as he is about his craft. The movie industry is fun, economical, fast. But novels are the thing.

"You are director, screenwriter, actors, make-up, the whole bang shoot. There is literally nothing else I would sooner do."