A professor of architecture at Cambridge University, Marcial Echenique, recently became curious when he found wiring concealed under the floorboards of his country mansion, Farm Hall in Godmanchester, Cambridgeshire. The mansion had an astonishing past as an MI6 staging post for some of the most daring secret operations of the Second World War. But in April 1945, Farm Hall was to play an even more astounding role, as a 'country club' for ten of Germany's...