Cover of "Sherlock Holmes: The Unauthorized Biography" by Nick Rennison
© Atlantic Books

I think it was just the strangeness of Holmes as a character, and the idea and vision of London that was created in the stories - the idea of Holmes as this very eccentric, striking figure, and a London filled with smoke and smog and Hansom cabs rattling down dark, cobbled streets. It was just very different from the suburban lower middle-class upbringing that I had.

You say in the book that the idea came from your editor, Angus MacKinnon.

Yes, it was his idea originally, but once he'd suggested it, it came to seem like a brilliant idea - to ask a sort of “what if?” question. What if Sherlock Holmes had been a real character rather than a fictional character? If that was the case, what sort of adventures in the 1880s and 1890s - apart from the ones which Conan Doyle had provided him with - would he have been involved in?

So you then looked for the juiciest scandals and most famous murder cases of the period…

It was then a case of going to the history books and looking at what was really happening in the 1880s and 1890s, which is the period when most of the best Sherlock Holmes stories are set. You then find that the sort of conventional image of late Victorian England as a very stable and peaceful society, with the empire on which the sun never sets, is in fact largely wrong. It was a time of great upheaval and there were all sorts of things happening - from assassination attempts on Queen Victoria, to Irish Nationalist bombing campaigns in London, to the Jack the Ripper killings. All of these are things that Sherlock Holmes, had he been a real person, would have been involved in - especially as his brother, Mycroft, was so close to power. It seemed fun to try and involve Holmes in them. So, really, the book is three strands woven together. There's the material from the Conan Doyle stories, there's the real history of the 19th century, and if those two failed to provide something then I was in a position where I could simply make it up.

Have you had any Sherlockians picking holes in your theories?

I was half expecting the green ink brigade to send me letters in which they pointed out that Holmes and Watson couldn't possibly have travelled to the West Country from Paddington Station on this date because there wasn't a train! We've had some slightly eccentric letters and emails but, by and large, it's been received in the spirit in which it was written - to try and write a straight biography. I didn't want it to be endlessly jokey. I wanted it to be read as if it was a straight biography of a real historical character.

One of your really striking ideas is that Professor Moriarty was an Irish Nationalist, and the brains behind the 1883 murder of Lord Frederick Cavendish in Dublin's Phoenix Park…

It struck me that, though Conan Doyle doesn't say anything about it, there's the very obvious point that ''Moriarty'' is an Irish name. I'd also been reading about the Irish Nationalist campaign for an independent Ireland in the 1880s; the two things coincided and I thought, well why not make have Moriarty born in Ireland and an Irish Nationalist?

Did you find it a problem knowing how to pitch the book - to a general reader or to a Sherlockian?

There is that, and when you've finished you're not absolutely sure yourself to what sort of an audience you've pitched it because, by that time, I'd read the stories over and over again so many times that I wasn't sure how far it would read to somebody who didn't know the stories at all. But I wouldn't expect anybody who didn't know the stories, or who hadn't got a significant interest in Sherlock Holmes, to pick the book up anyway.

Which actor was the best Sherlock Holmes - Basil Rathbone or Jeremy Brett?

I think Brett is brilliant. Basil Rathbone's films are fun to watch, but they're very, very dated. You know the story about how Brett became utterly obsessed by Holmes, towards the end of his life, in the later episodes, he almost began to imagine that he was really him. But in the earlier ones I think he's just brilliant. What he got so well, that other people hadn't got, is the sheer oddness of the character - that Holmes in the original stories is a very odd man. The fact that Watson is such a stolid, ordinary Englishman, or ordinary Briton, only emphasises that the more.

Have you ever dreamed about Sherlock Holmes?

I dreamed of finishing the book! It seemed as if it was unlikely that I ever would. But no, as yet the deerstalker hasn't appeared in my dreams! I think I'd begin to get a bit worried it it did!

• Sherlock Holmes: The Unauthorized Biography by Nick Rennison (Atlantic Books, 2005) is priced £8.99.