One September morning six years ago, Dick Francis was shaving in the bathroom of his home on the island of Grand Cayman while his wife Mary dressed in their bedroom. The couple had just returned from their annual trip to England, his latest book had been dispatched to the publishers and they were looking forward to the usual influx of racing friends fleeing the northern winter for Caribbean sunshine. Hearing a faint cry from next door, Francis put down his razor and went to the bedroom: in some distress, Mary, an asthmatic, asked him to fetch her inhaler. A few minutes later she died in his arms.

The Dick Francis story has been repeated many times: born on a horse, had a good war flying Spitfires and Wellingtons, rode 350 steeplechase winners, became champion jockey and was piloting the Queen Mother’s horse, Devon Loch, to certain victory in the 1956 Grand National when it fell eight strides from the winning post, making them the most famous losers in racing history. He retired from the turf at 36 and wrote a bestselling autobiography followed by 38 racing thrillers — one a year — delivering the last one in 2000, a month before Mary’s death. They had been married for 53 years.

It looked like the end of one of the most successful runs that a former jockey could hope to enjoy. Nearing 80, Francis had already agreed with Mary that this book should be his last; her death could only harden his resolve. She had been his helpmeet, researcher, editor and collaborator; they workshopped plots together in bed at night and travelled the world gathering material for the stories.

The previous year, an unauthorised biography had caused a minor furore by claiming that it was Mary, the English graduate, who wrote the books, not her famous husband who had left school at 15. The rumour was denied — “I write them in longhand, Mary types them out,” Francis told interviewers — but he never made a secret of his reliance on her and often said that her name should appear on the cover as co-author. After her death, who could blame a heartbroken Francis for shutting up shop?

Meanwhile, millions of fans worldwide faced a bleak future: we would have to do without the annual treat of the new Dick Francis; plane journeys would drag terribly without his latest page-turner. As my Times colleague Simon Barnes once perfectly put it: “Three Bloody Marys and a Dick Francis and you’re halfway across the Atlantic before you’ve noticed you’ve taken off.”

But joy! After six years of being obliged to resort to old favourites from the Dick Francis backlist for consolation (I have now read High Stakes four times), here we are, the master and I, sitting in a Devon hotel lounge with a handsome new hardback, Under Orders, on the table between us. “It’s so delightful to see this,” says the diminutive Francis, patting the cover. “I really didn’t think I would write again without Mary.”

The change of heart is down to the woman sitting demurely at his side, working on a piece of tapestry: Dagmar Cosby is the tall, slim, elegant 60-year-old from West Virginia who came into his life 18 months ago. His son Felix, who gave up work some time ago to manage his father's affairs, had been urging him to get down to another book: “Dagmar gave me inspiration,” says Francis.

So she made him feel, well, frisky again? “That’s exactly so,” he says. “She could never replace Mary — she won’t mind my saying that — but she has been a very good substitute.”

Dagmar smiles brightly. She is one of those American women who make you feel like a real slacker: impeccable make-up and hair, shining with health, perfect French manicure, and you know that she flosses. The love interest in the new book is a Dutch woman called Marina, much taller than the ex-jockey hero and obviously modelled on Dagmar.

“I didn’t look at another woman after Mary,” says Francis. “Then I met Dagmar at a weekend house party. She came down to visit me in Cayman and since then we’ve become very close.” Francis’s entire extended family is assembled at the hotel — a rendezvous that has taken place every summer since he and Mary moved abroad for her health: “He’s a younger man since he met Dagmar,” confides Mary’s sister. “We’ve all noticed it.”

Francis says that he asked his sons, Felix and Merrick, if they thought Mary would mind him having an affair with another woman. “They said no.” Does he think she would mind? He frowns. “I don’t know.” Well, would he mind if the situations were reversed? His brow clears and he smiles: “Oh no, I’d be pleased for her. I’d want her to be happy.”

He and Mary met at a wedding and afterwards, her sister tells me, Mary went home and announced that she had met the man she was going to marry. Neither family approved of the match: Francis’s father, a bloodstock dealer, couldn’t see how a girl who knew nothing about horses would be much use to anyone, and her family didn’t want her to marry a jockey.

But throughout his racing career Mary went along to every meeting, steeling herself for the inevitable falls: “Someone has to be around to pick up the pieces and drive them home,” she would say. Her husband put his shoulder out in crashing falls so many times that it would sometimes slip out when he turned over in bed and Mary learnt to put it back herself rather than call a doctor out. After a bad fall in a race at Hurst Park, the ambulancemen had difficulty manoeuvring the shoulder and had to send for Mary, who was waiting outside the tent.