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What makes a whistleblower? What prompts someone to break ranks, maybe break the law in order to expose a secret, often at great cost? Stephen Sackur interviews Katharine Gun. In 2003 she worked at the UK's signals intelligence agency GCHQ. She leaked potentially explosive information about America's covert effort to sway UN diplomats to support the Iraq war. She risked everything, including prison, in an act that changed her life. Now her story has been made into a movie; but 16 years on has her perspective changed?