Block C, Bletchley Park

Block C is located on the eastern edge of the Bletchley Park complex, immediately adjacent to and north of the new main gate. It is a spider block, single storey with the exception of a roof-top water tank room in the south-east corner. The building is steel-framed and encased in buff English bond brickwork. The roof is a reinforced concrete slab, slightly cambered and covered in asphalt. A group of glazed roof lanterns in the centre of the building are additions from the immediate post-war GCHQ occupation. The building is currently mothballed with most windows boarded up externally, though their scale and sloping red tiled cills remain visible. Internal inspection confirms that the windows are predominantly large, rectangular, multi-paned steel-framed casements with internal sloping white-tiled cills. Rainwater goods are plain and unobtrusive. To the east are two covered bicycle shelters, which are lean-to structures with roofs supported on square-section concrete pillars and housing concrete bicycle racks.

The main entrance is off-centre in the south elevation, approached by shallow concrete steps with simple metal hand rails. Solid wooden double-doors are inset into a metal-framed and glazed bay with yellow-tile detailing. Originally there were only four doorways: the main south entrance, pedestrian doors to Spur 2 (the administration spur) and Spur 9 (the machine bay), with a loading bay into Spur 5. Further pedestrian doors were added during the immediate post-war period, providing access to the ends of the other southern six spurs. A further large entrance with double doors is off-centre in the north elevation, where the flanking panels have been infilled in stretcher bond and a rain canopy is supported on brick piers.

In plan, the building is arranged around a central north-south corridor from which three pairs of west-east spurs branch off in the south and centre of the building, with an additional E-shaped arrangement of spurs with arms running north-south at the north end.

The main south entrance leads into a lobby with a further pair of double-doors opening into a large central room. This is the only surviving area of the largely open-plan wartime block, although it has been altered by the addition of roof lanterns and a suspended ceiling. The roof of the interior is supported throughout on steel piers against the external walls and brick piers in the centre of the building. The latter have chamfered corners and are painted white; those in the main common room are now encased in wooden cladding and others have been incorporated into later partition walls. The concrete beam roof construction is visible where suspended ceilings have not been added, such as in Spur 2. Original internal walls are stretcher bond using plain flettons, painted white. Most trusses are of steel construction in differing forms. Internal wooden doors are either solid or have glazed lights, all with simple architraves; some retain their original Bakelite handles. Post-war additions creating student accommodation include borrow-lights above onto the spur corridors. Bakelite switches and early cable trunking survive in parts. Cast iron radiators are found in many rooms and, as they appear in wartime photographs, are primary.

Off the central room are six spurs. Spur 1, in the south-east corner, was the kitchen and sanitary spur. Spur 2 in the south-west corner was the administration spur, originally subdivided to provide offices, kitchen and ablutions. Spur 2 is perhaps the most original in form of all the spurs, though some modifications were made when it was converted to GCHQ training accommodation in the 1940s. Spur 1 has additional reinforced concrete trusses supporting the roof-level water tower. Post-war conversion of the toilets to bath and shower rooms has altered the original arrangements. Spurs 3 (to the east) and 4 (to the west) were originally open-plan, housing the card store for completed jobs and the punch room and verifier bay respectively. They have been subdivided for training accommodation, though the original piers remain visible. Spur 5 (to the centre east) was partially open-plan in its original form and housed the new card store and a loading bay at its east end. Spur 6 was closed off from the central bay at its east end as it housed the engineering and maintenance facilities as well as two offices for senior engineers. These spurs have been subdivided in the post-war period, though the original piers and limited number of original 1942 subdivisions remain visible. The original spine corridor in the centre of the building, along with Spurs 7, 8 and 9 in the north (which were the main machine operation areas), have also been subdivided, though the original piers are visible. Part of the roof of the northern spurs has failed, allowing water into the building such that the suspended ceiling was collapsing at the time of designation in 2011.