Which country estate served as the headquarters for the British codebreakers during World War II?

It is in Milton Keynes . But it wasn’t in Milton Keynes . This is not an Enigma - at the time of the Second World War Milton Keynes did not exist . UK players will know that it was created in the 1960s as a New Town . When my children were young they called it Milk’n’beans

And what a marvellous and very interesting place it is too, plus the fact my Mother worked there!