Foyle's War creator reveals why he addressed the show's most famous gaffe in his new book

Six years later and Horowitz is addressing the mistake in his book The Sentence Is Death – his second novel to feature Daniel Hawthorne, a prickly private investigator with a shady past. Available now in paperback, it directly references the Foyle's War mistake in its first chapter, with Hawthorne stepping onto the set at the moment the error was made.

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"If you read the first chapter of the book, it is pretty much exactly what happened," Horowitz tells RadioTimes.com. "We were shooting a scene in London and the correct bus, which was an earlier [period] bus, had broken down. The props company, or the transport company, or somebody, then supplied a Routemaster, which was the wrong bus.

"The full story is that we were hoping at the end of the show we'd be able to use CGI and to turn it back into the correct bus, but we ran out of money!"

Why include the Foyle's War sequence in his book? "I love the clash between fiction and reality in the book, that Hawthorne himself turns up on the set and so you've got fact followed by fiction, intruding on each other," Horowitz explains.

"But also because I wanted to show that it wasn't just the stupidity of the writer, or the ignorance of the producer, or shaving money off the budget that caused this thing to happen. Making television is an incredibly complicated business and sometimes, when you're on a budget and things have to happen very quickly, things go wrong, and that was one of them."

The "clash between fiction and reality" is at the heart of the Hawthorne mystery novels, which, in an unusual twist to the genre, feature Horowitz himself as a character, with the author serving as sidekick to the lead character.

"My publishers asked me to do a series of murder-mystery novels," Horowitz recalls. "And my very first thought was, who's the detective? Should they be fat or thin? Male or female? Gay or straight? British or foreign? What ethnicity? What class? And I realised that every single idea had been done.

"There are robot detectives out there, vampire detectives... it's all been done! So then I began to think, what could I do that hasn't been done? And I had this idea of dropping myself into the narrative, which would completely turn things on their heads.

"I have certain rules. In a Sherlock Holmes novel, the books are not about Watson, they're about Sherlock Holmes. These books are not about me. I put some of my life into it, but mainly my writing life – it's not about where I go on holiday or what row I had with my wife!

"I have to be careful, because I don't want to read the book and think it's an ego trip, because it isn't."

Indeed, Horowitz says he's "much more interested in Hawthorne" than in the fictional Anthony who appears alongside the private eye. "Hawthorne is a much more interesting character than me – these are the Hawthorne novels and there is a lot to tell. I know this terrible backstory that made him what he is and I can't wait to tell it, although it'll take me another eight or nine books to get to the end."

The Sentence is Death by Anthony Horowitz is out now.