In 1951 a notebook filled with details about one of Britain's biggest post-war intelligence secrets was accidentally left lying on the floor of a London pub.

Its pages revealed the existence of Britain's top secret code-breaking centre at Eastcote, later to become GCHQ.

This centre carried on vital Cold War surveillance work that had been pioneered at the now famous Bletchley Park. At the time the public knew nothing about Bletchley or Eastcote.

Speaking to Today reporter Mike Thompson, Dr Christopher Moran of Warwick University explained: "It is only today that we can actually discover just how close… [we] got to revealing what was, without question, the biggest post-war secret."

Radio 4's Document programme reveals how close that secret, which survived for another 23 years, came to being revealed.

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