It wasn't really a film about code breaking, it was a film about social attitudes to homosexuality. A few of the facts were altered within the bounds of dramatic licence: Turing, for example, started working on the Enigma machine before the war and not after.

Hugh Whitemore's portrayal of the controlling nature of the government was good. Because, interestingly, during the actual war itself - when the secret was so precious - there wasn't very much control at all. It was only afterwards, as the Cold War intensified, that they began to become much more paranoid. There's no doubt that they were terribly worried about Turing, Burgess and MacLean, because Turing knew more secrets than the government did. You have to bear in mind that he didn't only work on codes: he developed a computer and he developed the trans-Atlantic scrambler that Churchill and Roosevelt used. He knew everything, really.

I think Whitemore wrote Turing very well. But it has to be said that, despite Derek Jacobi being a brilliant actor, in 1939 Turing was 27 and only in his early forties when he died; Derek Jacobi is now 58. There was a scene when Turing's been out running - he was a fantastically good cross-country runner - and there was old Jacobi puffing away. He spoke like an old man, he moved like an old man: you didn't get any sense of a rather shy, gauche young man of that generation.

I met Turing's girlfriend, to whom he was engaged for a time, who drew me a plan of where they all worked. She went on after the war to work in codes and didn't leave GCHQ until the 1980s. There's still a lot of secrecy - she was still very careful about what she would and wouldn't say.

Robert Harris wrote the bestselling novels 'Fatherland' and 'Enigma'.