After penning Sherlock Holmes novel Moriarty, Anthony Horowitz is set to tackle James Bond

Horowitz has also been chosen by the Ian Fleming Estate to write the next Bond story, following in the wake of the likes of Kingsley Amis and William Boyd

On a roll: Author Anthony Horowitz (Jason Alden)

Anthony Horowitz is on a serious roll. Sixty next year, though he could easily pass for forty, the novelist and script-writer – creator of Midsomer Murders and Foyle's War on TV as well as many adventure stories for children – is writing better than ever. He is also the father of two sons who still live in the family home (but in their own apartments). We are not meeting there, though, but in the London offices of Eleventh Hour Films, an independent company founded by his wife and co-worker, Jill Green.

This month sees the publication of Moriarty, a page-turning novel for all ages that continues the story of Sherlock Holmes's greatest enemy, after what now appears to be his faked death at the Reichenbach Falls. Horowitz has also been chosen by the Ian Fleming Estate to write the next James Bond story, following in the wake of the likes of Kingsley Amis and William Boyd.

How on earth does he manage to keep producing such quantities of highly readable stories, all so ingeniously plotted? "It's because I love writing now as much as I did when I was a child and doing it for fun. It always has been and remains a joy as well as a physical pleasure. I adore the scratching of a pen." "A pen?" "If I am following another author I always work in the way they did. So a fountain pen for Conan Doyle, and a typewriter for Ian Fleming."

Horowitz's previous Sherlock Holmes pastiche, The House of Silk, while still celebrating the Baker Street duo, contains several sharp criticisms of their characters and working methods. Sending out the Bow Street Irregulars, Holmes's willing ragamuffin army, in search for clues to a dastardly crime now no longer seems such a good idea when one of them is brutally killed as a result. Always a butt for condescending scorn in the original stories, Inspector Lestrade from Scotland Yard, who warns against this practice, is allowed to emerge as someone of moral integrity. A pity he was not there to hear Holmes most uncharacteristically remarking: "It may be that Lestrade was right."

Holmes is not directly criticised in Moriarty, as he only appears in an imitation short story tagged on at the end, with good old Watson once again taking over the narration. For the rest of the novel the great detective remains in hiding after what he hoped had been the final dispatch of the arch-villain he most feared. But his habit of instantly diagnosing the lives and habits of people he has just met is once more called into question by Horowitz – this time when it is adopted by Holmes's faithful follower Inspector Athelney Jones. He is another Scotland Yard detective for whom Holmes had little time in the original stories. On the trail of Moriarty as well as some American criminals, Jones teams up with Frederick Chase, who admits to being a Pinkerton agent travelling incognito as part of the hunt. But Jones's skills are more severely tested than ever was the case with Holmes.

Horowitz has always adored Doyle, and this novel is crammed with references to some of his best-loved stories. For Horowitz, "without Holmes there would have been no Foyle". But what is it about the detective genre that has led him to write so many whodunnits, with more still in the offing?

"I saw Anthony Shaffer's play Sleuth as a teenager with my parents. I remember saying to my father in the interval, 'I don't think that chap is really dead. I think he's going to come back'. And my father replied, 'That's the most stupid thing I have ever heard.' But I went on obsessing for years afterwards about this wonderful play and all the possibilities it opens up for writers fascinated by the crime genre but who also want to do something original with it."

"What did your father say at the end?"

"My father was a strange man. He said nothing at all." Strange indeed. A financier and one of Prime Minister Harold Wilson's occasionally dodgy business associates, Horowitz's father faced bankruptcy and moved his assets into a Swiss numbered bank account before his death. Despite every effort since, the money remains untraced. There were other family tensions too, with one grandmother so horrible that Horowitz and his sister quite literally danced on her grave once she had died. Overweight, lonely and finding lessons hard at his boarding schools, he turned to writing as a retreat and then realised that this is what he wanted to do for the rest of his life. Horowitz's favourite villain in Moriarty is a murderous fourteen-year-old boy called Perry, in revolt from his own public school. "I empathise with him!"

But writing still remains very demanding, often entailing getting up at 5am and working through the day. Even his sleep is interrupted by thoughts about plot details or the direction of a story. So why take on James Bond, such a different character from both Holmes and Foyle? "I really loved the James Bond books and films as a boy. Queueing up in the rain in Leicester Square for three hours to be among the first to see Live and Let Die was pure pleasure. Without Bond there would have been no Alex Rider.

"But I don't want to write another Holmes story. I've done that. I now want to have a crack at Bond and, when I have finished that, I want to write more thrillers – but this time taking readers into areas they may not have been to before. In a year's time I will finish The Magpie Murders, which I hope will prove to be the whodunnit to end all whodunnits. Everything to do with the central mystery, including its solution, is contained in its first three pages, but no-one I have shown them to so far has ever got near cracking it. Readers are going to find their faces slapped so hard they will not believe it. But having spent thirty-five years writing about more standard murders, clues and red herrings, it is time to try something new."

He is now off for a six-day holiday in Crete, working but also enjoying the sun. A good thing his airline doesn't charge for the weight of all the other stories currently held in his head but still to be written. If it did, Horowitz would need an excess baggage allowance like no other.

Anthony Horowitz's 'Moriarty' is published by Orion on 23 October (£19.99)