Historians say that no more than 30 people ever knew of its existence. Its role was to intercept, with the help of the Post Office, foreign correspondence. A staff of 10 included experts at breaking seals and forging new seals on diplomatic letters.
Correspondence written in code, was sent by personal courier, to the grandly named Deciphering Branch of the Department of the Secretary of State. In reality, this was a family business begun in the 18th century by the Rev. Edward Willes and then taken over by two of his sons and three of their nephews.
They cracked coded messages and devised new ones. They were well rewarded and became an extremely wealthy family.
Max Glaskin is an award-winning freelance journalist regularly published in the Sunday Times, New Scientist and many other national publications.