As we become ever-more aware of how our governments "eavesdrop" on our conversations, here is a gripping exploration of this unknown realm of the British secret service: Government Communication Headquarters (GCHQ).

GCHQ is the successor to the famous Bletchley Park wartime code-breaking organisation and is the largest and most secretive intelligence organisation in the country. During the war, it commanded more staff than MI5 and MI6 combined and has produced a number of intelligence triumphs, as well as some notable failures. Since the end of the Cold War, it has played a pivotal role in shaping Britain's secret state. Still, we know almost nothing about it.