I am so sorry,” says a stripy-socked Anthony Horowitz, apologising that his dog has greeted me first as I climb the three flights of stairs to his central London flat. “I was deeply immersed in a murder.”

He hastily dons shoes and makes coffee, musing, “If I’d known you were coming from Golders Green, I would have asked you to bring some smoked salmon…”

The homicide preoccupying Horowitz takes place in The Word Is Murder, an adult novel due out in September and now at the final edit stage. But first comes the launch of Never Say Die, the latest Alex Rider novel for teenagers. If you’ve been on the moon for the past 17 years (and that’s no excuse — Alex has been into space) Rider is a teenage secret agent. The last Rider book, Scorpia Rising came out in 2011, with a prequel, Russian Roulette, in 2013.

“I didn’t mean to go back to Alex,” says Horowitz. “I really had meant to quit. I wrote Russian Roulette to bring the series full circle.” But then he was asked by Walker Books to compile an anthology of Rider short stories.

“I realised how much I’d missed Alex and that I still had lots of good ideas. It persuaded me I should do another novel and — between thinking that and starting work on another novel — one day passed!

“I felt [at the end of Scorpia Rising] I’d left Alex far from home, on his own, depressed and part of me thought I owed it to readers to return Alex to himself, to make him happy again.”

There will definitely be more Rider adventures. The start of the next — Nightshade — is built into the end of Never Say Die. Rider is more relevant than ever, says Horowitz, as terrorist threats pile up on us. So which antagonist would the boy spy tackle first?

Probably none of them. “Alex doesn’t have a world view, Alex is a teenager. He wants to get on with his life. What makes Never Say Die a successful book is he’s not out to get the bad guys, he’s out to…” — I’d better edit the sentence here, for fear of spoilers — “… Alex never sets out to save the world.”

Another facet of Alex’s youth is, he is never allowed his own gun. “When Alex is in action, a lot of people tend to die,” says Horowitz. “But he seldom pulls the trigger himself. The Alex Rider books are meant to be fun, escapist… I don’t want to get too close to the bone. It’s interesting how, when you write a series, it gets darker and darker. It happened to J K Rowling. I’m glad Never Say Die returns to the lighter atmosphere.

“The reason Alex ended up so damaged in Scorpia Rising was that he had shot another boy. It was quite a difficult moment to write.” And it is no small thing for Horowitz to use the word “difficult” about writing. “I always stay away from the word,” he says. “If the writing’s difficult, something’s gone wrong. Writing is like breathing. If you say breathing is difficult, you need a hospital.”

For once, in Never Say Die, the villains are low-key. Rider’s enemies are usually larger than life — with a deadly jellyfish as a pet, a scarred face, a warped pseudo-scientific ambition or a highly original murder method, such as burying an adversary alive in coins. But Never Say Die sends Alex on a more personal quest. “I deliberately chose a plot that wasn’t world domination — it’s a much more intimate book for that reason.”

Rider books have exotic locations, and in Never Say Die, we visit Egypt, the Cote d’Azur and Oxford. Will Rider visit Israel one day?

“He should,” says Horowitz. “I don’t know why he hasn’t.” But then he reflects: “There is a danger of getting too close to reality… it would be difficult to weave a fun-filled fantasy into it. But Jerusalem is the most exciting place on the planet — the energy! The confluence of history and religion and blood and torture and endless struggle is such a huge story…”

In a technological age, Rider’s Bond-like gadgetry still relies more on fantasy than techno-wizardry (watch out though for geeky, Jewish Johnny Feldman in the new book). Horowitz collects mechanical toys and prefers these to techno-gizmos:

“I like the fact that Alex is a child and there’s a childish element to keep levels of violence down.”

On the necessity of including violence, one thinks of the words of Damian Cray, the lead villain in Horowitz’s Eagle Strike: “We did develop a game where the hero had to collect flowers from a garden and then arrange them in vases… But do you know what? Our research team discovered modern teenagers didn’t want to play it.”

Rider’s uncle brought him up with the skills of a secret agent. Did Horowitz’s parents set out to raise a writer?

“It’s a source of sadness to me that my father not only didn’t believe in me being a writer but thought I was incapable,” says Horowitz, quietly. “I regret that he never saw my success. My mother did believe in me and did tell me stories as a child and died just after my first son was born. She always said I based my villains on her, which wasn’t true!”

In reviving his hero, Horowitz is following his own example of Conan Doyle and Ian Fleming — having written official sequels for both of them.

“I feel almost nervous about tiptoeing back into the world of children’s books — I have learnt so much from Doyle and Fleming,” he says. “Books like Magpie Murders and The Word is Murder are a real step forward. It’s nice to be writing after 47 books and realising I’m not drying up, I’m getting better.”

He began work that day, he tells me, about 6.30am and expects to finish around midnight — seeing my jaw drop, he says quickly: “with a gap! I have a social life! I go to the cinema! I have friends, dinners… The most sensible thing Margaret Thatcher said was never underestimate how much you can do in an hour. And I have about nine or ten hours a day alone.” Even when he is walking Boss, his Labrador/Staffie cross, he thinks about work. His wife, Jill, a television producer, works with him on many of his series.

“I never stop. I don’t consider myself disciplined. But I love what I do. All that matters is the moment when I take out a fountain pen and begin to write.” He does not use a computer until the second draft.

“With The Word Is Murder, I set out to write a new series — and I began by finally acknowledging I had to write whodunnits.” This is something he had already done for TV. The idea of a straight whodunnit exasperated him: “23 chapters that lead up to the butler did it. Why bother?

“I started looking at the relationship between writer, detective and sidekick — and I thought: how can I twist it?” He decided to get rid of the sidekick and take his place. “So I am the sidekick … I’m the Watson. It transforms the approach. I love how, even though I am the author of the book, I can’t solve the crime.”

Magpie Murders sold more than 100,000 copies in the UK and comes out in America this week.

“It took me 15 years to write,” says Horowitz. “I wasn’t good enough. It was a complicated book and I needed to be a better writer to do it.”

Is he implying adult literature is greater, more important than children’s? Horowitz is appalled at the suggestion. “If you ask me what is the writing I most value and want to be remembered for when I die, it is Alex Rider. The number of parents and teachers who say Alex Rider got [their child or pupils] into reading! Getting young people involved in books is critical. Books are a rock, they are a foundation.”

He doesn’t rule out a return to writing for the stage, undaunted by the mixed reviews received by his first play in 2015. Dinner With Saddam was a comedy about Saddam Hussein’s habit of dropping in to dine with ordinary citizens.

“The whole cast was terrific and I thought it was a useful play in that it looked at Iraq in a different way. I was dismayed that some critics were so dismissive but theatre is a world unto itself. It doesn’t take kindly to interlopers. When I write another play I may have to write it anonymously. I’m not sour but I’m puzzled by it and I want to crack it.”

Coming soon are another James Bond book, the TV version of Magpie Murders, an Alex Rider TV series and the Rider short story collection. But if he could do only one thing, what would it be? “Looking back — it would have to be children’s books. Looking forward — it would be adult books. You have to grow.”

At 62, he loves to meet fans who are in their 20s and remember being avid Rider readers when they were 10. He enjoys “the sense that I have been one molecule in their bloodstream throughout their formative years. When I started writing Alex I had children in the house [sons Cass and Nick are now in their late 20s and run their own media company] and I’m further away from my audience with every year that passes. But the child in me never dies.”

Never Say Die is published by Walker Books on June 1