Next Friday sees the cinema release of Sherlock Holmes: A Game of Shadows, the follow-up to Guy Ritchie's 2009 blockbuster starring Robert Downey Jr as Holmes and Jude Law as Watson – a cavalier re-imagining of Sir Arthur Conan Doyle's fictional detective for the age of the multiplex. Viewers willing to accept fiction's greatest violin-playing sleuth as an unkempt, prize-bareknuckle-fighting slob and the stolid Watson as more kick-arse than Jason Bourne have presumably never been anywhere near the source novels – or if they have, they will have a greater taste for absurdity than your average Holmesian purist.
These aficionados need only hang on until the New Year, however, for the return of Sherlock, BBC1's witty and deeply knowledgeable contemporary reworking of the stories. The three new 90-minute dramas, once again starring Benedict Cumberbatch and Martin Freeman, may be set in the here and now, but they are in 100 ways more true to the novels that Ritchie's lavish Victoriana.
In the company of Gatiss (who plays Holmes' brother Mycroft) and his Sherlock co-creator, Steven Moffat, I'm snooping around the living room of 221B Baker Street – not the real one, of course, because that never existed, at least not in Conan Doyle's day. This particular 221B – a bachelor-pad clutter of books, empty bottles and crisp packets alongside test-tubes, a chess set and a music stand – has been constructed in a warehouse on an industrial estate on the outskirts of Cardiff. It's right next door to where Doctor Who is filmed, in fact, and the connection between the two shows is far from coincidental, it being on the train between London and the Welsh capital that Doctor Who writers Gatiss and Moffat started discussing their love of Holmes, dreaming of a modern-day version, and sharing their esteem for the 1940s movies starring Basil Rathbone and Nigel Bruce.
Rathbone and Bruce in the Forties were followed by Peter Cushing in the Hammer Holmes films of the Sixties and Jeremy Brett 20 years later. And now, after another two-decade hiatus, we have the simultaneous Downey Jr and Cumberbatch incarnations.
Screened during the summer holidays last year the show was an overnight success, trending on Twitter, debuting with nine million viewers, and being praised by the Culture Secretary, Jeremy Hunt, in the House of Commons. Its fan base is worldwide, Sherlock having been sold to 180 different countries (Downton Abbey sells to about 100).
What people thought would never work, of course, was updating the Sherlock Holmes stories to the 21st century, with Sherlock as a freelance police adviser and John (Watson), an army doctor newly returned from Helmand province in Afghanistan.
Cumberbatch and Freeman are beautifully cast, their characters correlating with the personalities of the actors (Freeman laid back and down to earth; Cumberbatch more garrulous and intellectual), although the latter is quick to distance himself from Holmes. "Because I talk a lot... probably because I'm nervous and can't quite edit the thoughts into something a bit more pithy... I get pinned into the same mania bracket, or having the same level of energy... I really don't. I'm very lazy in comparison to Holmes and I operate at a far lower speed."
Shortly after we meet, both actors are due to fly to New Zealand to appear in Peter Jackson's movie of Tolkien's The Hobbit (Freeman as Bilbo Baggins; Cumberbatch as Smaug). Jackson, it transpires, is a big fan of Sherlock, as is Steven Spielberg. "Spielberg came down on set," says Cumberbatch. "He went nuts for it."
The new trilogy of 90-minute films begins with "A Scandal in Belgravia" (an adaptation of the first Holmes short story, "A Scandal in Bohemia"), followed by "The Hound of the Baskervilles" and is rounded off by "The Reichenbach Fall", a version of "The Final Problem", the 1893 story in which Holmes seemingly tumbles to his death during a fight on an Alpine ledge (this story also forms part of the new film).
In the editing suite I am shown the first 15 minutes of "The Hounds of Baskerville", and, without giving too much away, it involves Russell Tovey (Him & Her), a Porton Down-style chemical weapons and research facility, and an escaped mutant animal. "It's trying to find a way of making a monster believable now," Gatiss says.
'Sherlock' returns to BBC1 in the new year. 'Sherlock Holmes: A Game of Shadows' is on release next week