It would take one of teenage ‘super spy’ Alex Rider’s state-of-the-art gadgets to penetrate the high security entry system guarding the London penthouse apartment of his creator Anthony Horowitz.

Once through two sets of heavy double doors and a lift journey to the third floor, a courteous Horowitz, 55, welcomes me into the living area of his home.

In this huge, open plan space, a flat screen TV, funky neon wall art and double bed-style lounger intermingle with classic pieces including an old piano and a bookshelf brimming with Ian Fleming first editions.

Horowitz himself is a mixture of old and new. Private school manners, traditional values and a confidence so often seen among people brought up in privilege, as he was, effortlessly combine with a visible love of all things trendy and modern.

He’s working on the ninth and final Alex Rider novel,

Scorpia Rising, which will be published next April.

"If I appear twitchy it’s because I’ve got Alex into a very difficult situation which I need to go and get him out of," he explains.

Up another floor and we arrive at his office, a long, light airy room with stunning views over the capital, where he shows me his latest acquisition – a gun, disabled of course, which he bought because his last Alex Rider novel will see the reluctant young hero use a gun for the first time. Horowitz wanted to know exactly what it felt like.

The Alex Rider series (the most famous of which, Stormbreaker, was made into a film starring Alex Pettyfer, Mickey Rourke and Bill Nighy) has sold more than 12m books in Britain and been translated into 28 languages.

"It’s the right thing to do," he reflects, on his choice to end the series. "It makes me sad and I’m sorry, but I’m not a machine churning out a product. I think writers owe it to themselves to keep moving on and to stop while they’re ahead.

"I’m a great fan of Ian Fleming and James Bond, but if you look at the last four or five Bond novels, they get worse and worse and just taper out – and I saw the same thing could happen with Alex.

"I’d run out of chases, out of bad guys, gadgets and reasons to want to blow up the world, and I thought, limit yourself."

"So is Alex going to die in the end?" I ask tentatively, looking at the gun.

"It’s worse," says Horowitz gravely. "When you read the book you’ll see very definitely that it’s the end. As desperate as I am to give him a happy ending, I can’t find it. It’s not there."

But his young fans have nothing to fear. Scorpia Rising will be followed by a book about the Alex Rider anti-hero, Yassen, and Horowitz has just written a collection of horror stories entitled More Bloody Horowitz.

Parents may baulk at the stories involving murderous game shows, a slaughtered publisher and a robo-nanny, but the

author says the grisly episodes are tongue-in-cheek and filled with humour.

"I like horror for young people. There’s no point in being completely violent, bloody and unpleasant. It’s written with a smile."

He’ll also be writing one more in the Power Of Five children’s series and one more

Diamond Brothers novel, which will take him to 60. After that he’ll stop.

Horowitz, who has also written extensively for TV, hates being pigeonholed as a ‘children’s writer’.

"It sounds too much like a party entertainer and I don’t see myself as that."

When he has completed the young series, he wants to write an adult novel, a play, a film and more television.

Indeed, he’s no stranger to TV work. His screen writing includes Foyle’s War, Midsomer Murders and, more recently, the ITV series Collision.

Horowitz is currently working on a new five-part series for ITV, a thriller about the dilemma faced by a barrister who knows the person he’s defending is guilty.

It will be filmed this autumn and his wife, Jill Green, will be producing the series.

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