A new film about Alan Turing, with Sherlock star Benedict Cumberbatch playing the grandfather of computing, has opened the London Film Festival.
The Imitation Game tells the story of the brilliant Cambridge mathematician, cryptoanalyst and pioneering computer scientist who spearheaded the Enigma code-breaking operation during World War II and was later persecuted by the British government for his homosexuality.
Alan Turing is played in the film by Benedict Cumberbatch, famous for his role as Sherlock in the TV series of the same name. He hopes the film will introduce more of the public to the remarkable man and his achievements that remained secret for half a century and long after his death in 1954.
"Having had some experience with this extraordinary man, I really want his story to be as widely known as possible," he said. "Why is this man not on some denomination of our currency?"
Norwegian director Morten Tyldrum said: "When I read the script, I was taken back by how little I knew. Why isn't he on the front cover of the history books at school?"
Cumberbatch talked about the difficulties of working out how to play a character who is not very well documented. "It was a blank canvas to some extent. You have a bit of freedom but you had nothing to bounce off as a reflection. Physically there's no media - ironically because of what he was about or maybe not, because of the secrecy - but there are no visual or audio recordings of him."
Instead, he had to rely on accounts from people who'd met him. Kiera Knightley, who co-stars as the fellow Bletchley Park cryptoanalyst Joan Clarke, said she had a little more source material for her character, including an interview recorded when she was in her seventies.
In modern terms Clarke was a feminist, said Knightley: "She was breaking boundaries in her own right but she didn't go about it like a bull in a china shop."
The cast said they had tried to understand the mathematics behind the code breaking but admitted they hadn't understood the briefings from experts at Bletchley Park. "I understood a lot about the Enigma machine – the actual coding machine – but if you put an algorithm in front of me now or even a quadratic equation this press conference would go on for far too long," said Cumberbatch.
"One day, we all thought we should all do the quick crossword, and there were five of us, but it took us five days," said Knightley. "So, no, we're really bad and I didn't understand any of the maths!"
Cumberbatch said when he had an explanation of the Bombe at Bletchley "that was the moment when I thought this was very hard".
However, he referred to the "great broad romance to the philosophy of maths and physics which is dramatised these days which is tangible. I think there are hugely exciting things on a base level that everyone can understand: the idea of coding, the idea of programming, the idea that what we use as a language can be turned into something universal and can be used in a machine here, China, New York, Russia, wherever."
Cumberbatch was asked whether audiences might think his new role too similar to that of his other great starring role as a clever logician - Sherlock in the TV series. "I am limited by who I am and what I look like," he said, "but he doesn't swish about in a cloak and curly hair demonstrating how brilliant he is. He's very quiet, stoic and different...I didn't read the script and go 'Oh, this is Sherlock in tweed.’”
Turing was given a posthumous royal pardon in December last year for his conviction for homosexuality – or an act of ‘gross indecency’ – in 1952, which led to his chemical castration and also saw his security clearance being withdrawn for his post-war work at GCHQ. He went on to commit suicide in 1954.
London Film Festival director Clare Stewart said: "The Imitation Game does cinematic justice to Alan Turing's vision, determination and personal story as well as his enduring impact on British history and contemporary life."