MI6.

History In 2010, the BBC World Service broadcast the documentary MI6 - A Century in the Shadows, whose title is pretty self explanatory. Nevertheless:

"An unprecedented look inside MI6 - Britain's Secret Intelligence Service, which marked its centenary in 2009."

"For this three-part series, BBC security correspondent Gordon Corera talks to senior intelligence officers, agents and diplomats as well as their former arch enemies about the shadowy world of espionage."

Here are links to the still available episodes for convenience. The BBC's own structure is less convenient. Podcasts are available:

Part One - Gadgets & Green Ink

The first part of this series explores the early years of MI6 - set up by Sir Mansfield Cumming, an eccentric and formidable figure known as 'C' who signed his name in green ink.

The highs and lows of the two world wars and details of some of the gadgets any self respecting agent could not do without.

Part Two - Heroes & Villains

The second part in this series describes what went on behind the Iron Curtain during the Cold War.

Former MI6 Chief John Scarlett describes his clandestine meeting with an agent and the Russian defector Oleg Gordievsky talks about his reasons for coming over to the other side.

Part Three - New Enemies

What is the role of MI6 in the 21st century?

The former head of Britain's Secret Intelligence Service talks about the interrogation of terrorist suspects and MI6's role in the run up to the war in Iraq.