Keira Knightley, as Katharine Gun, stands nervously in the dock where she is asked ‘Do you plead guilty or not guilty?’ - and the screen goes blank.

This is how Official Secrets starts. It is the true life story of Katharine Gun, the GCHQ worker who leaked a memo to the press about the pressure that the US was placing on smaller members of the UN Security Council, to pass a UN Resolution to allow the US (and Britain) to go to war against Iraq. The US did not get its Resolution – but went to war anyway.

This film, though, mainly follows Katharine: her agonising over her action, her decision to confess, and all the events leading up to that appearance in the dock. Matters are complicated by the fact that she is married to a Turkish Kurd who is seeking asylum in the UK. The film also follows the secret path whereby the smuggled document is leaked to the Observer, their checking of facts leading up to the printing of the story (and Spell Check plays an important part here too!) and all the machinations afterwards: these are flashed through fast as viewers get to grips with the characters (Real life names and their titles appear on the screen when necessary.) It is quite a roller coaster. Then, there is the mind-numbing period while Katharine waits to see what the government will do; the small and not-so-small humiliations she is subjected to, her sadness that she has not stopped the war and saved lives and yet the growth in her belief that what she did was right.

Keira Knightley is absolutely convincing as Katharine Gun – capturing her fear but also her anger and her determination. Matt Smith as the young reporter Martin Bright - part cocky, part shyly pleased with what has been achieved; Rhys Ifans as an outspoken but dedicated and hard-working Ed Vulliamy; Ralph Fiennes as the Liberty lawyer – they are all excellent. The smaller parts – Tamsin Grieg as Elizabeth Wilmshurst, Matthew Goode as reporter Peter Beaumont for instance– are all terrific too. It is indeed an all-star cast.

Katharine Gun’s bravery is still extolled by many, including other whistleblowers, but she is hardly a household name, as for instance Edward Snowden is. This film rightly gives her the credit she deserves for her principled stand, despite the potential cost to herself.

‘I work for the British people. I do not collect information so that the Government can lie to the British people.’