Two years of ill health, which began with major heart surgery and ended in a leg amputation, have not deterred Dick Francis from racing ahead with another surefire bestseller – with the help of his son, Felix.

The author is now 87 and, although visibly frail, looks dapper in a smart shirt and tie for our interview. He is a perfect gentleman of the old school.

His enthusiastic son Felix, 55, is slightly more casual and much more vocal, constantly filling in any memory lapses and completing stories for his father. However, underneath it all I suspect that Mr Francis Senior, former Spitfire pilot, champion steeplechase jockey and award-winning author of 41 novels, is as tough as old boots and quite able to answer for himself.

His page-turning equestrian thrillers, including Driving Force, Shattered, Longshot and Proof, have been translated into 33 languages and each one goes straight to the top of the bestseller list. Now the two have collaborated on their second novel together, Silks, a cracking read in which the hero, a barrister and amateur jockey, defends a fellow jockey he believes has been framed for murder.

Felix and Dick jostle verbally like any father and son; the quiet stubbornness of the wise old man versus the protective, well-meaning attitude of the concerned son.

Dick laments that since he had his right leg amputated below the knee, after suffering circulatory problems following a heart bypass, he has been unable to drive. Felix suggests that's a blessing in disguise.

"He drove like he rode – see a gap and go for it," he says.

Felix, a former physics teacher who has managed his father's affairs since 1991, last year appeared for the first time as joint author of Dead Heat and has now collaborated on Silks.

"This book should actually be called 'Hospital Rooms We Have Known'," says Felix, referring to that triple bypass surgery his father had the day after his 86th birthday.

"The heart surgery took a long time to get over, then he tripped over and broke his pelvis, which involved another stint in hospital. Then last summer he had an unfortunate incident which resulted in him losing his foot.

"After surgery his swallowing wasn't great. He went out to dinner and choked. If he hadn't been having dinner with a heart surgeon, he would have probably died. The surgeon managed to resuscitate him but in the melee his foot was badly bruised. There was insufficient blood supply to the foot for it to recover.

"It was very traumatic. He was first in hospital in Cayman, then in Miami and then in London. I literally carried him on and off the aeroplane."

Surgeons managed to save the knee and now Dick has a new prosthetic leg and is pleased to be walking again, albeit with the help of a metal walking frame.

"He complains that he can't walk with this and carry a glass of red wine," Felix quips.

"Everyone tells me I look very well," Dick pipes up. "I've been fine. But it is frustrating. Having lost my leg, I know my life is limited from now on."

What does he miss about not being as mobile as he was?

"Riding six races in an afternoon, I expect," Felix laughs. But he is obviously relieved that his father now has a full-time carer, an ex-nurse they have known for 18 years, at his home in the Cayman Islands. The carer not only provides physical help but is a companion who has in some ways helped to assuage the loneliness he felt when his wife of 53 years, Mary, died in 2000.

Returning to writing has helped filled the huge gap Mary's death has left, Dick admits. There's at least one more book on the cards, which he and Felix have already started.

"Felix has taken over a lot of the ideas that Mary and I used to discuss. It has increased my telephone bill."

Felix continues: "I ring up and he says, 'I'll call you back', because that's what dads do."

It was long mooted that university-educated Mary wrote the books herself and Dick agrees she had huge involvement, with her great knowledge of the English language and her fastidious research. She learned to fly a plane for Flying Finish and Rat Race, learned about painting for In The Fray, about photography for Reflex and about computers for Twice Shy.

However, it was a partnership, Felix insists. For years Dick had asked her to put her name jointly on the books, but she'd refused.

"It was always a joint effort. They were like Siamese twins conjoined at the pencil," he says.

Ironically, the couple made a decision to retire in 2000 with the publication of Shattered.

"Blow me, no sooner had we said that, then Mary had a heart attack and died. That left me absolutely stunned," Dick explains.

When asked if he went into depression after his wife's death, Dick says firmly: "I'm not one for being depressed, but I've missed her terribly."

Felix interjects: "He did go into a bit of a depression. He was just existing, so his agent and I hatched a plan. I took him to the Melbourne Cup and to Dubai to see the Godolphin horses and to Hong Kong and it brought him out of it."

As a top steeplechase jockey who rode the Queen Mother's horse Devon Loch to near victory in the 1956 Grand National (the horse fell just short of the winning line), Dick is no stranger to pain.

"Ask him which bones he's broken," Felix instructs me.

"Quite a few," Dick retorts. "But I never broke anything below my waist. I was most upset when they told me I had to lose my leg."

"He's broken his collarbone six times on each side," Felix continues, "his nose five times, he's cracked his skull, he doesn't count ribs."

The index finger of his right hand is permanently curved outwards, a legacy of breaking his wrist years ago.

Today, Felix takes a much more active role in the writing. The pair discuss ideas, Felix researches a subject, he taps out the story and emails it to his father, who amends it. Is Dick Francis doing an editing job, then?

Dick nods, Felix doesn't. "He's far more involved in plot development and also style," he says.

This is Dick's third book since Mary's death and the process has become easier with Felix's help, admits Dick.

"It took me a long time to get into it. I'd wonder, 'What would Mary think of this?'"

Felix continues: "When we argue, the answer is always, 'Well, your mother would have made me do that!' The ultimate decision of what goes into the book is what he thinks she would have said."

Mary is buried in Grand Cayman but, due to a mix-up, the plot reserved for her husband has been taken by someone else.

Dick laughs about it now, as he reveals that he'll now be buried underneath a pretty path which runs along the other side of her grave.

"I'd never leave Cayman because Mary's buried there and I don't want to leave her."

Felix recalls the story behind his mother's gravestone epitaph, "Come on, my darling".

"Dad hates being late. He would always be standing at the door and my mother would say, 'I'll just go to the loo'. He would roll his eyes and say, 'Come on, my darling', in exasperation. My mother used to say, 'That'll be on my grave'. And it is."

Dick has already decided on his own epitaph: "I'm with you now, my darling."

Silks by Dick Francis and Felix Francis is published by Michael Joseph/Penguin, priced £18.99. Available on Thursday, September 4.