A tiny old man in a large Oxfordshire kitchen is neatly writing postcards to America despite the fingers misshaped into question marks. There are five others in the room – his son Felix, Felix’s girlfriend, Felix’s girlfriend’s daughter, a carer and a photographer – but Dick Francis, the octogenarian royal jockey turned novelist, never stops writing; nor does he look up to inspect another arrival until he’s completed the address, right down to the zip code.

I begin to understand the methodical drive that takes a man from winning post to top of the bestseller list, time after time. Every year, after he finishes a novel, he begins his Christmas cards, sent in aid of the Injured Jockeys Fund, and fires off letters in between. His light frame is now frail. Last year he had a triple heart bypass and a foot amputated, yet he’s still the most commanding person in the room. He signs his postcards “Legless Dick”.

A walking frame and a wheelchair are close by. The eyes are red-rimmed but still sparkling, and it is easy to believe, on meeting this charmer, that one of the only four times he fought with his beloved wife, Mary, was after he spanked her bottom while she was cleaning the grate.

“It’s a great honour to meet you,” I say, blushing as I take the gnarled hand, tanned from his decades of living in the Cayman Islands (I have been warned not to mention the impending arrival of Hurricane Gustav in case I make him anxious).

For me, this is an honour. Not only have I brought in my bag a copy of his gripping new thriller Silks – written with Felix, his younger son – for him to sign, but I’ve also been round my house gathering up paperbacks for the master’s autograph too. Just as the biographical blurb memorably used to read: “Dick Francis can’t remember learning to ride: it came as naturally as learning to walk”, I can’t remember learning to love a Francis book: it came as naturally as falling off a log.

I say “Francis” rather than “Dick Francis” because everyone present – from Felix, a former physics teacher, who also co-authored the previous novel, Dead Heat, to Dick and Felix’s girlfriend Debbie – is keen to stress that the writing is and has always been a collective effort, even if Felix was not credited in their first collaborative effort, Under Orders. Dick Francis may be in a league of his own, but he is also 87, and while he looks bronzed and says he feels horribly well, it is clear he tires easily and feels the cold of our glorious English summers like a knife through his turquoise polo shirt.

Felix, who has managed his father’s affairs since 1991, explains: “The whole Dick Francis brand has been the family business. My father is called Richard Francis, and my mother was Mary Francis [Mary is famous for having carried out meticulous research for the books, to the extent of learning how to fly for Flying Finish and Rat Race, taking up photography for Reflex, and learning to paint for In the Frame]. The two of them together were Dick Francis. It’s always been a cottage industry, but without the cottage.”

Still, I am here because the world knows the name Dick Francis, an author who has sold 75m-plus books, whose intricate, go-faster plots have been copied by betting syndicates and who will for ever be remembered for failing to win the Grand National in 1956 when Devon Loch, the horse he was riding for the Queen Mother, slipped to the ground 50 yards from the finish.

While I do not detect any rivalry between the father and his son – in fact, I witness a miraculously close working relationship – I do glimpse an oedipal shadow cross Francis Jr’s sunny face just once, when I insist on miking up his father, not him, for sound.

It has to be said that during the interview the lion’s share of the talking is done by Felix, with interjections from his father in a low rumble. Felix finishes his sentences and takes questions for him. When I ask: “What did the Queen Mother call you?”, Felix answers: “She called him Francis in the 1950s, when everyone was known by their surnames, but apart from then she called him Dick.”

And how did you address her, I persist. Your Royal Highness? “Your Majesty,” Dick Francis replies, sitting up straighter at the table. “But everyone in racing always referred to her as Queen Elizabeth, except the Queen, who called her Mummy.”

“Everyone in racing calls the Queen Brenda,” adds Felix.

“I thought Private Eye invented Brenda,” I say.

“No, racing did. People will still say at a race meeting, ‘Is Brenda coming?’ ” Felix insists.

“Please don’t make it all about the royal family,” Dick says quietly.

How could I? I’m sitting here with possibly the greatest living Englishman (born in Wales) and one of the most successful living authors in the world, and I want to go on about the royals? Not likely!

I want to know what it’s like writing a book with a close family member, and how his latest collaboration with Felix took off. Felix says: “I got a call from our agent and he said, ‘We have a problem. There’s this backlist of wonderful Dick Francis stories but no one is reading them; we need a new Dick Francis novel. We need a new frontlist.’ ” At this point, Felix relates how he hesitated: his father, then 84, had been through two years of hospital hell and bereavement, and hadn’t produced a book for four years.

“Then the agent said, ‘I’ve got someone in mind who might write one’, and I said, ‘If anyone’s going to have a go, I’d rather it was me.’ I said I would do for my father what Mum had done in terms of research. It took 18 months, but Under Orders was the result – though the title should have been Hospital Rooms I Have Known,” Felix concludes.

“And I was more than delighted,” murmurs Dick.

I am perplexed. How on earth could a father and son write a book together when one lives in Oxfordshire and the other in the Cayman Islands, and neither likes Skype?

“The telephone bill was absolutely huge,” Dick mutters, but the process was that Felix would send a page, Dick would print it out and then they would discuss it.

Felix bounced between their houses like a yo-yo.

Okay, but what about the violence and the injuries that all Dick Francis heroes have to sustain on a page-by-page basis? Both admit that Dick Francis heroes, all of whom are in some way injured, disappointed, lame or wing-down, are based on Francis père. Did the two men have the same approach to character, to violence and to, er, sex?

At the mention of physical pain and injury, Dick perks up. “As I’ve got older I’ve become no less violent,” he says cheerily.

“Yes, the Queen Mother did once complain you were getting too bloodthirsty,” Felix reminds him. “But the truth, Dad, is that the books are about what you’re about. Loyalty and courage. Not sex and violence.”

They both sit on the sofa, father and son. It is true. The books are about endurance and doing the right thing, almost as much as they are about what happens to a body when it hits the ground at 30mph and then a horse falls on top of it.

As I get up to ask Dick to sign his books, and Felix and Dick to sign Silks, he says: “I used to go back to Cayman on my own after Mary died, and I was lonely. Now we’ve spent so much time discussing the books, and writing them, it’s been a great help to me. A new lease of life. How long I shall go on, I don’t know.”

A long time, we all hope. My bet is that the Francis franchise is here to stay.

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