The 79-year-old novelist spotted Mokhiniso, known as Niso, as she was walking in London's Sloane Square. She was, in Smith's words, an exquisite, gorgeously nubile Asian creature. "I couldn't take my eyes off her. Such a cute bottom," he says, miming her hips with his hands. "Everything, but everything, was in the right place and moving so perfectly."
When Smith spotted Niso, she was walking into a branch of WH Smith. The author followed her in and watched as she browsed the bookshelves. "She picked up a Ken Follett, or maybe it was a John Grisham, and was clearly going to buy it," he says. "So I went over, took her hand and said, 'This is what you should read,' showing her the wall of Wilbur Smiths.
"She told me she was a student and her tutor had recommended she buy a long book to read aloud to improve her English," he says. Smith showed her his author photograph, bought her a book, although he can't remember which one, signed it, then invited her to lunch.
"I knew she wouldn't say no, she was a starving student. But she wasn't that impressed when I said I was a published author. In her country, writers are the worst paid in society so she told me later she thought, 'This poor old man.'"
Smith demonstrated the truth by taking Niso, who is 39 years younger than him and from Tajikistan, near Afghanistan, for a slap-up lunch at one of Knightsbridge's smartest restaurants, where they ate lots of caviar. They married in 2000. "We've been lunching ever since," he says, his face lighting up. "It really was love at first sight – and now she's got the best English teacher in the world. Of course people ask about the age gap, but I just say, 'What's 39 years?' Sure, she's young enough to be my daughter, so what?
"The only time we get problems is if we visit Moscow, where Niso trained as a lawyer. There, older women will shout abuse at her in the street, 'What are you doing with that old man? How much is he paying you? You're a whore!' Which is very distressing for this lovely, young woman, who has mended my broken heart."
What had broken Smith's heart was the death from brain cancer of his third wife, the writer Danielle (Dee Dee) Antoinette Thomas. They had been married for 28 years and she was the love of his life.
"I never thought I'd ever meet anyone again, let alone fall in love and marry," he says. He was 67 years old and feeling desperately lonely without Danielle – "although I should tell you, because of her illness, it had not been a true marriage for more than six years". There was no sex? "No, there was nothing physical – and we had always been very loving, but she became very sick," he replies.
Smith met Niso just a few weeks after Danielle's death and in the dedication in his latest novel, Those In Peril, calls her the queen of his heart (she returns the favour in every one of her emails, signing them: "Wilbur's soulmate, playmate and accountant"). He sheds years when he talks about her although he doesn't look that old – he could pass for sixty-something at most. The only sign of fragility is a pronounced limp, the legacy of childhood polio.
Smith and Niso have no children and he's not close to his three children from his first two marriages. He has grandchildren – he's vague on how many – and a stepchild, Dieter, Danielle's son from her first marriage. Niso chose to remain childless so they could go on indulging their passion for travel, which he combines with the meticulous research that fills his books: skiing in Switzerland, fishing in Iceland, hunting and shooting in Botswana, lunching and shopping in London and New York. "It was her decision," he says when we meet in London. "I was happy to accommodate her either way."
Today, the literary lion is in a rented townhouse just across the street from Harrods, while their palatial home in Chelsea is being refurbished. "None of this is ours," says Smith, gesturing at the bland, featureless furnishings, lest I think they have no taste despite the fact they are rolling in money.
They are doing up their London house because they plan to spend more time there. Apparently, Niso likes the shops and he's decided London's where it's at. They will, though, keep their Cape Town house, Sunbird Hill, on part of the original Cecil Rhodes Estate, in the lee of Table Mountain. Recently, they have downsized, selling their game reserve, as well as an island Smith owned in the Seychelles, where he found the inspiration for Those In Peril, a bloodthirsty tale of Somali pirates which continues to give me recurring nightmares – I fear page 289 will stay with me for ever, I tell him.
At this point, the elegant Niso – her famous derriere clad in designer denim – dashes into the room, full of charm and abject apologies for interrupting. She dashes out again almost immediately but not before chiding her husband for not offering me so much as a glass of water. "You are a terrible host," she exclaims, while Smith signs some urgent paperwork for a "Chelsea chariot" he's buying. "She's always wanted 'one of those big cars that English ladies drive around London,'" he explains, indulgently.
Actually, Smith could buy her a fleet of gold-plated 4x4s if she wanted. He is a publishing phenomenon: the global No.1 bestselling author of 33 books, which have sold 120 million copies around the world and been translated into 26 languages. It's estimated that with world rights, each time he writes a book it makes him around £4m. He is Pan Macmillan's most successful author and has 22 of the publishing house's coveted Golden Pan statues, awarded to authors whose books exceed one million sales. His nearest rival received only seven – he was Ian Fleming, the creator of James Bond.
So who buys Smith's books, these gory tales of derring-do, stomach-churning violence and lurid lust in the dust that appear to have been written in testosterone-soaked ink?
"Women," he exclaims, with pride. "Women love my books. Of course men do too. But it pleases me that so many women of all ages like my work. I know this because they write to me. And, yeah, schoolboys enjoy them, too."
Indeed, London Mayor Boris Johnson is a huge fan. When he was at Eton, he claims Smith was "venerated for his dog-eared sex scenes". Tattered paperbacks of Smith's oeuvre would circulate round the dorms, according to Johnson, always handily falling open at the pages of most relevance to adolescent boys.
If young Etonians are still poring over Smith's novels, then Those In Peril will not disappoint. It features the craggy, uber-manly ex-SAS major Hector Cross, who is as good between the sheets as he is at saving the world. US-based Reelart Media has bought the film rights, with the movie scheduled to be released in 2014, the year Smith reaches half a century as a novelist. The producers believe Cross has the potential to be a 21st-century equivalent to Bond, Robert Ludlum's Jason Bourne or Tom Clancy's Jack Ryan. The sequel to Those In Peril, Vicious Circle, also starring Cross, is already written. "It'll come out in 2013," promises Smith, who says writing is as addictive as cocaine. "Not that I've ever tried it."
Set in present-day Africa, Those In Peril begins with the spoilt daughter of a ball-breaking female oil tycoon being captured by Muslim pirates in her super yacht. The girl is sexually enslaved to the man who was once her lover but who – surprise, surprise! – turns out to be the vicious leader of the pirates. Her mother, who is of course drop-dead sexy and filthy rich, and Cross – he of the rippling muscles – plot her safe return while getting it together.
The book's up-to-date setting is unusual for Smith, whose books have ranged from the Egypt of the Pharaohs to colonial Africa sagas about the feuding Courtney and Ballantyne dynasties from the 1600s to the 1980s. Legions of fans are hooked on these novels in which elephants are forever being shot and "lots of good stuff" happens, by which Smith means lashings of sex.
In Those In Peril, however, Smith wanted to write a stand-alone book about a mother being deprived of her adored child, then having to get help from one of his tough guys. "I wanted her to be tough, too," he says, "because I'm a feminist." Then he recalls that when he owned his island in the Seychelles, where he had a big boat, he and his crew once came across Somali pirates.
"They didn't acknowledge us – rare among sailors – but I've never forgotten this guy, tall and very handsome, as hypnotic as a black mamba, just standing there. His eyes were dead. I guess I was lucky they weren't kidnapping that day; I could so easily have been taken, but that image has stayed with me. I knew I would use it one day."
With Smith being a born storyteller, the immediacy of the subject appealed to him too: in 2011, $159.6m was paid in ransom money to Somali pirates, including $13.5m for the release of the Greek-owned tanker Irene SL. In all, 1118 hostages have been held. On average they are kept for 178 days, but some for much longer. During the past year more than 120 attacks were foiled, but maritime piracy still costs an estimated $12bn to the global economy.
As Smith points out, the pirates are developing new tactics, dramatically changing the rules of the game.
"They're using sophisticated radar equipment and offshore accounts. I think the disturbing thing about Somalia is the fact this is a country where there are few economic opportunities, so the pirates are seen as glamorous, and often held in awe by young boys who aspire to their lifestyle," he says. He shakes his head in despair.
Trained as an accountant, the lifestyle Wilbur Smith aspired to was always that of a successful writer, one who wrote the kind of books he likes to read – the sort that Stephen King, one of his keenest fans, calls swashbuckling novels where the bodices rip and the blood flows.
The son of British parents, Smith was born and raised on his father Bert's 29,500-acre ranch in Northern Rhodesia (now Zambia) and educated at Michaelhouse in Natal, South Africa – "the Eton of the veldt" – then at Rhodes University, where he did a degree in commerce.
His father did not believe in wasting time reading, let alone writing. "He was a Victorian," says Smith, who nonetheless worshipped his father. "He was the moon, the sun and the stars to me. He did things; he raised cattle, he built roads, he built buildings. No, I don't think he ever read any of my books." Smith dedicated his first book, When The Lion Feeds (1964), to him. "I don't think he read it, but he carried it around to show to his friends."
On his eighth birthday, Smith was given a 22 Remington repeater rifle by his father; he shot his first lion when he was 13 – or rather, he shot a lion and two lionesses after they killed eight of his father's Brown Swiss cattle. The lion charged the boy, then the lionesses followed suit, so he shot all three. Nonetheless, he's a conservationist, a trustee of the World Wildlife Fund and has never killed an animal or a bird he doesn't eat. These days he no longer hunts. "I can't run any more," he says, indicating his gammy leg.
As a boy he wanted to become a journalist but his father told him he'd starve so he became a tax assessor for the Inland Revenue instead. He jokes that he wrote his first book on Her Majesty's time – and on her paper. In 1964 he married Jewell, but they later divorced; then he married Anne, but that marriage was also dissolved.
The marriages, which are dismissed in a sentence on his website, produced three children: Shaun, who went into the Rhodesian SAS before becoming a businessman, a daughter, Christian, and their half-brother, Lawrence. Nothing is known of any of them. In 1971, Smith married Danielle, adopting her son Dieter from her first marriage. His relationship with his children has been profoundly troubled, but he refuses to discuss it. He named Dieter, a specialist doctor, as his sole heir. Then, after his mother's death and his stepfather's fourth marriage, Dieter unsuccessfully sued Smith, airing the family's dirty linen in the process, according to the South African press.
"He's gone," says Smith. "Gone for ever. To America. It's not far enough," Smith said last year.
"They all go their own way," he tells me. "They get on with leading their lives; I get on with mine. Neither they nor my grandchildren are a major force in my life. Niso's the centre of my universe. She's everything to me, so I'm safeguarding her future. She will inherit everything one day. I have to make sure she is taken care of."
Not that he spends much time thinking about death. His father lived to be 80; his mother until 95. "I reckon I've a good 10 years ahead of me. Anyway, I'm living life to the full; this is the best time of my life. I've never been happier."
Then his phone rings. It's Niso, his soulmate, playmate and accountant – she's got her wheels. n
Those In Peril by Wilbur Smith is published by Pan, priced £7.99. The author will be in conversation with Jackie McGlone at the Edinburgh International Book Festival on August 25. Visit www.edbookfest.co.uk.