He's the Peter Pan of pathology as eccentric Dr Donald 'Ducky' Mallard in the hit US crime series NCIS, now in its 13th season.

Indeed, at 82, Scottish-born actor David McCallum, best-known to the older generation as sexy Russian spy Illya Kuryakin from cult Sixties series The Man From U.N.C.L.E., seems to have as much energy as someone half his age, and has no intentions of retiring.

"Let's not go too J.M. Barrie," he quips at the Peter Pan reference, "but I've never really shaken off a certain boyish enthusiasm for life. It's kept me going."

This charming octogenarian has now branched out into writing, and his debut novel, Once A Crooked Man, is a gripping yarn about a family of mob bosses who decide to retire from crime and tie up some loose ends (in other words, bump off some of the firm who know too much), but are hampered by an actor who intervenes after inadvertently overhearing their plans.

McCallum wasn't forced into reinventing himself as an author. Indeed, his role in NCIS has reportedly made him one of the highest-earning British TV stars in the US. He's also gleaned quite a knowledge in pathology throughout the series, and still frequently calls the coroner for information. "I've learned a lot and also have at least six feet of death books on my shelves."

He's also had his share of tragedy away from the spotlight, most notably the death of his adopted son Jason, who died from an accidental drug overdose in 1989.

"You never come to terms with the death of a child," McCallum reflects. "The pain is very real, but it's like an ache that turns into anger.

"At the same time, there's a frustration that you couldn't really have done anything about it, which makes it even worse. In the final analysis, he had the life he had. You just have to accept it."

He and first wife Jill Ireland had adopted Jason and had two other sons together, but divorced when she left him for Charles Bronson, who she'd met while he and McCallum were filming The Great Escape together. They remained in California, while McCallum moved to New York.

"I never went through a feeling of, 'What if I'd have done this?' with Jason, because when I divorced Jill, she got full custody of all three of my boys," he adds.

He has a good relationship with his other two sons from that marriage.

"Val, the youngest, is a musician who plays lead guitar with Jackson Browne, and my son Paul [a studio photographer] has a beautiful daughter and they're not far from here, I see them all the time," he says proudly.

McCallum went on to marry interiors designer and former model Katherine Carpenter, with whom he had two more children.

To some, their 49-year marriage might seem unconventional, as for most of the year they remain in different states - he in Los Angeles, where NCIS is filmed, and she in New York, the base for her work.

"We've spent 10 months of the year apart for the last 13 years. She is part of the oldest interior design firm in the US and works out of New York.

"We keep in constant touch and there are all these vacations in America, like Thanksgiving, when we are together. And cellphones give us a constant communication."

The grandfather-of-eight flies back and forth to his permanent home in New York, which gives him a few hours to work on books.

So, what's the secret of a happy marriage?

"The great thing is to do everything as an individual that you need to do to further your career, who you are and what you are. But when it comes to marriage, we do it instinctively, but I think the rule is you should find out what the other person wants in life, and make sure they get it," says McCallum.

"My wife at this point would like to become two people - one that would be here in California and one there in New York, and we both feel that way. We're reaching the point after 13 years that we should probably think about getting together."

But he has no intention of quitting NCIS - he's signed up for this season and hopes to carry on next year.

Yet he admits he and Katherine need to try to arrange more down time, at least to enable them to see more of each other, and their children and grandchildren.

"They call me Grand-daddy. We don't like Grandpa because it sounds old - and I'm not old," he declares. "And some of them have the cheek to call me Grand-Ducky!"

Born in Glasgow, the son of very musical parents (his mother was a cellist and his father led the London Philharmonic Orchestra), McCallum attended the Royal Academy of Dramatic Art, where Joan Collins was a classmate, and was initially primed for a career in music.

After national service, he found parts in radio plays, and later in British films including 1963's The Great Escape, before clinching his role in The Man From U.N.C.L.E., which made him a star on both sides of the Atlantic and reportedly gained McCallum more fan mail than any other actor in MGM's history.

"Once here, I just loved the life in California. When we did U.N.C.L.E., we were flavour of the month," he recalls. "I couldn't go out in public without being very careful. We got mobbed and attacked. I once got rescued from Central Park by two mounted policemen who rode me out through the park. It was a wild time. "

He returned to Britain spasmodically, appearing in Colditz and cult sci-fi series Sapphire & Steel with Joanna Lumley.

Today he lives in New York state, keeps in touch with his old pal Robert Vaughn, although they rarely meet up, and is modest enough not to make much of his enduring celebrity.

Yet recently he was mobbed by U.N.C.L.E. female fans in Macy's in New York, as he turned up to sign albums in which he conducted instrumental versions of new and old songs. The store had to close off Herald Square so that police could drive him away safely.

Looking much younger than his 82 years, McCallum doesn't really think about ageing, says he hasn't had cosmetic surgery - and wouldn't publicise it even if he had.

"As long as I can keep moving and look OK on camera and sound pretty good, I can get away with NCIS. I do Pilates twice a week when I can, and I try to avoid all forms of stress. I don't eat red meat and I have a good diet. I don't eat as much as most people."

Away from the acting spotlight, he's eager to continue his new-found literary career and has been commissioned to write a second novel.

"Whenever I have trouble sleeping now, I think, 'Well what would happen next?' Instead of counting sheep, I'm counting new characters."

:: Once A Crooked Man by David McCallum is published by Sandstone, priced £8.99. Available now