Block D (which I didn't visit) at Bletchley Park was constructed for the occupants of Huts 3, 6, 8 and the Hut 15 group, all of whom were short of space. As these sections worked so closely, the obvious solution was to bring them together in one building and they moved into Block D in February 1943.
After the move, 6IS (SIXTA) found that it was still short of space. This section was building up a picture of the German radio network. In Autumn 1943, it moved into a new building, the two-storey section of Block G. The single-storey section of Block G was occupied by the Intelligence Service Knox and ISOS (Illicit Services (Oliver Strachey)) sections which dealt with German Secret Service material. The strategic importance of these sections increased in the lead up to DDay, as by accessing Abwehr material, the Government Code and Cypher School could assess the success of deceptions perpetrated upon the Germans.
Later on the building was used by BT.
Thanks for looking,
Rubex
After the move, 6IS (SIXTA) found that it was still short of space. This section was building up a picture of the German radio network. In Autumn 1943, it moved into a new building, the two-storey section of Block G. The single-storey section of Block G was occupied by the Intelligence Service Knox and ISOS (Illicit Services (Oliver Strachey)) sections which dealt with German Secret Service material. The strategic importance of these sections increased in the lead up to DDay, as by accessing Abwehr material, the Government Code and Cypher School could assess the success of deceptions perpetrated upon the Germans.
Later on the building was used by BT.
Thanks for looking,
Rubex