It becomes a watchable but rather flat reworking of true events that played out behind the scenes prior to the invasion of Iraq in 2003.

Director Gavin Hood (Rendition, Eye in the Sky) retells the story of British "spy" Katharine Gun (Keira Knightly). She worked as a translator at Communications Headquarters (GCHQ) in Cheltenham when the Blair government was deciding how to support American foreign policy as the US was spoiling for another confrontation with Saddam Hussein.

A minor player, Gun made the difficult - and illegal - decision to leak to the media a top-secret email that was issued to intelligence employees. The document came from the chief of staff of the American National Security Agency, asking for British help in digging up dirt on members of the United Nations Security Council so that pressure could be applied to get the war vote across the line.

It was one step too far for Gun, who famously stated: "I do not gather intelligence so the government can lie to the British people."

The film follows Gun as she struggles with her conscience and then - after the memo is published by The Observer - as she deals with the legalities and politics of the Official Secrets Act.

It's an elegant and poised movie, if somewhat unimaginative. Expect the standard clandestine meetings in underground carparks and by-the-book newsroom arguments about whether the editor should run that controversial story.

It's also a very talky affair, frequently getting bogged down in careful explanation: of the politics behind the war, of the meaning of the leaked memo, and of the legal options available to Gun.

The first half of the story focuses on Gun and her decision to leak the email, with consequences for her husband Yasar (Adam Bakri), a Turkish Muslim with uncertain migrant status in Britain.

We also meet Martin Bright (Matt Smith) the reporter who breaks the story, and his flamboyant colleague Ed Vulliamy, (a rather over-the-top Rhys Ifans).

The second half of the film follows the legal story, Gun taking advice from human rights lawyer Ben Emmerson QC, played with almost dull restraint by Ralph Fiennes.

In fact, everyone is far too polite and restrained: one of the most dramatic scenes in the film concluding with a request for a nice cup of tea.

Hood seems caught in a balancing act: unwilling to dive deep into the emotional side of Gun's private world, and not prepared to take us close to the messy politics that were driving the forces against Gun.

Rather, we find out about the unbelievably dramatic manoeuvres of the British government in a quiet scene where legal advisor to the Foreign Office Elizabeth Wilmshurst (Tasmin Greig) drops the ethical bombshell to Bright without a hint of emotion.

Blink and you'll miss what should have been compelling drama.

Much of the problem lies with the patchy screenplay, which Hood reworked from a script that did the rounds for years. It was by Sara and Gregory Bernstein, from Marcia and Thomas Mitchell's novel The Spy Who Tried to Stop a War.

Whistle-blowers have a fascinatingly ambiguous status, especially when they work for secretive government agencies.

Enemies of the state or heroes of the people?

Either way, their stories are rich with intrigue and drama.

It's disappointing then that, despite strong performances from Knightly and Smith, this is a reserved English affair.