Keira Knightley on Bletchley heroine Joan Clarke

Joan Clarke, the only female codebreaker to work alongside Alan Turing and his team at Bletchley Park, recounts the moment the mathematician, who was gay, proposed to her.

Clarke helped to crack some of the most difficult Navy Enigma ciphers during WWII and the intelligence she gathered saved countless lives.

Her interview in this BBC Horizon programme, from 1992, is one of the only instances in which she spoke about her time as a cryptanalyst.

Clarke is played by Keira Knightley in The Imitation Game, a film about Turing's efforts during WWII.