H Block
HISTORY: Block H was the last significant building to be erected at Bletchley Park in WW2. Plans were approved on 25 May 1944: it was ready for occupation on 17 September 1944. It was built on the western edge of the site as an annexe to the now-demolished Block F (pulled down in 1987), which was located immediately to the east, and its purpose was to work on the deciphering of the 'Fish' series of encrypted German teleprinter transmissions. The large machine rooms at the southern end were built to house six Colossus Mk 2s (Nos 5 - 10 at Bletchley Park): this was necessary, given the volume of signals being intercepted, and the pivotal role of Bletchley Park in the overall Allied intelligence-gathering process. The Colossus computer was developed in 1943 by Tommy Flowers, based on concept designs by Max Newman.

SUMMARY OF IMPORTANCE: Block H is architecturally undistinguished, being a utility design incorporating a standard Ministry of Works 1942 hut design (see The Builder, 28 August 1942,176). Nonetheless the building has strong claims to international historical note as being the world's earliest purpose-built building erected specifically for electric computers (Block F, now demolished, included an annexe designed to house computers but Block H was designed from the outset to serve this purpose). The Colossus is regarded as being the world's first programmabLe electronic digital computer: Bletchley Park witnessed the earliest mass-installation of computers, and is thus a key site in the development of information technology. It also forms part of the ensemble of surviving wartime buildings at Bletchley Park which, through their deciphering of encrypted Axis messages, made a significant contribution to Allied victory in the Second World War.

C Block

Block C was built in 1942, and thus forms part of the mid-war phase of expansion at BP, which witnessed the construction of purpose-designed brick buildings intended to supplement the Victorian mansion, outhouses and huts that had been used at the start of the war.

The function of Block C (which had formerly been located in Hut 7) was to house the Hollerith section. Hollerith machines were punch-card operated index machines, developed in the 1900s, used for storing information from earlier decrypts of encoded German messages. The machines were manufactured by IBM and were in use internationally.

The resulting Intelligence Index system, presided over by Freddie Freeborn (and thus referred to colloquially at BP as the 'Freebornery'.), was an important component element of BP's highly effective system of intelligence gathering. By drawing on earlier decrypt analyses, numerous possible permutations of the Axis codes could be ruled out, thereby speeding up the process.

The building was planned in 1941 but construction was delayed because of delays in obtaining building materials (notably steel). Once completed, the scale of the operation was considerable: some 2 million punched cards a week were being produced, indicating the sheer scale of this part of the operation.

The building was subsequently used for a period as a training school by GCHQ from 1946 to 1987: this involved extensive subdivision and internal alteration. Block C has languished in more recent years.

D Block

INTERIOR: the central spine corridor has several flights of concrete steps along its length to allow for the sloping site, but is relatively unaltered. In spite of subsequent alterations, much of the wartime lay-out of Block D can be appreciated through fabric and documentary analysis. Each northern spur had an off-centre corridor with offices to either side, these corridors were lit by transom lights over the office doors. Offices were austere internally, with walls of bare brick, Some hatches between offices survive in place. The southern spurs had larger Watch Rooms which have now been subdivided or re-configured. All traces of belt conveyors and pneumatic tubes, used for swift internal communications, have gone.

Spurs A, B and C retain the strongest sense of their wartime character; Spur H is of particular associational value because of the location of Welchman's office. Spurs 0 and C in particular have been much altered internally.

HISTORY: Block D was originally planned for about 530 persons; by the war's end, some 700 were working here. Its main function was the vitally important one of breaking, deciphering and analysing German Enigma coded traffic. This work had previously been undertaken by Huts 3,6 and 8. The building of a large bespoke block to take over these functions represented the development of the scale and effectiveness of the code-breaking operation at Bletchley Park. The details of exact usage of Block D are complex (see EH report, 412 ff.). Hut 3, tasked with reporting on Wehrmacht and Luftwaffe Enigma, moved into Block D in February 1943 and occupied Spurs A,B,C,G,H and I. The reception point and Watch Room of Hut 3, located in the spine offices between Spurs H and I, are some of the most important areas of Block D. Hut 6, responsible for deciphering this Enigma traffic for Hut 3, occupied Spurs D,E,J, K and H. Gordon Welchman, one of the key figures at Bletchley Park, had his office at the northern end of Spur H. Hut 8 prepared decrypts of navaL Enigma (which was separately analysed and handled), and occupied Spurs F and L. The intelligence gained here played a key part in ensuring Allied victory in WWII.

ASSESSMENT OF IMPORTANCE: Block D was considered by GCHQ to have been "the most important block in the park" (EH report, 392). Its claim to special interest is primarily historical, and this is considerable. In its scale and planning, the block clearly demonstrates the development of Bletchley Park's intelligence operation, from the extemporizing temporary accommodation erected here at the start of the war, to a more planned and greatly enlarged approach to signals intelligence process. Decryption and signals analysis activities within this building had a direct bearing on the successful prosecution of the Allied cause. Block D also has claims to notice on visual, planning and associational grounds. The largest component block at Bletchley Park, it is probably the most historically important spider block to remain anywhere and thus represents a once-common type of wartime building. Architectural impressiveness is confined to the entrance, but the whole structure is outwardly evocative of wartime demands for rapidly built accommodation, and inwardly significant because of the activities that took place here. It also forms a significant part of the overall Bletchley Park complex, a site with very considerable historical value as the principal centre for gathering and disseminating signals intelligence during WWII.

The above are extracted from the Listing documents prepared by English Heritage based on the
report by Linda Monkton et al, 'Bletchley Park' (English Heritage Historic Buildings Report B/101/2004: 4 vols 2004)