Block C, Bletchley Park


Heritage Category:
Listed Building
List Entry Number:
Date first listed:
Statutory Address:
Block C, Bletchley Park, Sherwood Drive , Bletchley , Milton Keynes, MK3 6EB


Ordnance survey map of Block C, Bletchley Park
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Statutory Address:
Block C, Bletchley Park, Sherwood Drive , Bletchley , Milton Keynes, MK3 6EB

The building or site itself may lie within the boundary of more than one authority.

Milton Keynes (Unitary Authority)
West Bletchley
National Grid Reference:


Block C, Bletchley Park. Spider block designed in 1941 to house Bletchley Park's Hollerith Section and operational from November 1942. GCHQ training school from 1946-1987.

Reasons for Designation

Block C at Bletchley Park is listed at Grade II for the following principal reasons: * Historic Interest (code-breaking): a building that housed the Hollerith section (Bletchley’s punch card intelligence index, machinery and staff) which was key to the success of Bletchley Park’s Second World War code-breaking work, specifically recording decryption information for the German Enigma code; * Historic Interest (The Information Age): a building highly significant in the development and use of machinery for mass data processing, where not only was the Hollerith system employed on a hitherto unprecedented scale but the machines and systems were secretly and constantly improved and adapted; * Form and fabric: a purpose-built and designed utilitarian structure which was hardened and soundproofed in order to protect the highly important machinery and data inside. It is one of the largest purpose-built buildings for tabulating machines in the world. Later alterations for a post-war GCHQ training school, while of lesser interest, are part of the story of the continuing intelligence use of the site at the end of the war; * Group value: with other contemporary buildings on site which collectively express the scale of the intelligence operation at Bletchley Park during the Second World War, some of which are designated.


Bletchley Park is a site of international historic significance for signals intelligence (Signit): a Second World War code-breaking hub and a highly significant site in the development of high-speed and mass data processing. The site retains a large number of historic buildings, many of which are listed, which collectively express the rapid technological developments on site through their fabric. Bletchley Park’s story, its historic fabric and significance has been written about in numerous publications including Winterbotham’s 1974 book The Ultra Secret, one of the first to reveal the significance of the wartime work here, and more recently English Heritage’s research report (2004). Therefore only a summary history will be provided here. Needless to say that Bletchley Park was a hugely significant Second World War intelligence ‘factory’ with nearly 9,000 people employed here by 1945 and it was here that the German Engima code was famously broken.

In 1937 part of the Bletchley Park estate, centred on the Grade II Victorian mansion house, was bought by the Government to house combined intelligence services previously accommodated in London. This included members of the Secret Intelligence Service (the SIS, also known as MI6) as well as Foreign Office staff from the Government Code and Cipher School (GC&CS, renamed post-war as Government Communications Headquarters or GCHQ). Initially operating from the mansion, additional accommodation was soon needed and a series of temporary wooden huts had been erected and fitted out by the summer of 1939.

Rapid expansion of the Bletchley Park complex led to plans being drawn up from mid 1941 onwards for a series of additional purpose-built and permanent blocks. While based on Ministry of Works temporary office block designs, each individual building was designed for its specific purpose in conjunction with the Bletchley staff. The first of these, Block A (Grade II) was operational in August 1942. Block C, which is under assessment here, was planned in 1941 and was first occupied in November 1942. It was designed to accommodate the Hollerith Section, led by Freddie Freeburn of the British Tabulating Machine Company Ltd., previously housed in Hut 7 (demolished post-war). As the Hollerith machines were very noisy and operated day and night both Hut 7 and its successor, Block C, were located away from other accommodation on the eastern edge of the Bletchley Park site.

Herman Hollerith (1860-1929) was an American inventor who gave his name to his system of recording information on punched cards which was initially used to analyse U.S. census data in 1890. His system allowed the analysis of vast amounts of data by machine therefore considerably speeding up a statistical process previously carried out by hand. In 1896 he founded the Tabulating Machine Company (which would become part of IBM in the 1920s). The Hollerith system was essentially an early form of computer processing.

Block C was purpose-designed and built to house the Hollerith machines and staff at Bletchley Park, providing an essential and central service supporting the whole of the GC&CS: the Hollerith system managed Bletchley’s punch card intelligence index and was therefore key to the success of its codebreaking work, specifically recording Engima decryption information. The Engima code machine enabled codes to be changed every 24 hours simply by resetting the wheels in the machine. The challenge for the staff of Bletchley Park was to break the code before the wheels were re-set. Using the Hollerith machines to analyse the decryption information (whereby the punch cards based on the coded messages would either block or allow electrical currents to pass through them) enabled the operators to limit the number of possible Enigma wheel-settings and therefore the number of solutions to each days’ Engima code. In addition, the Hollerith section used its machinery to advise on Allied ciphers.

The Hollerith machines were also adapted on site. This was a critical part of the way that the research here was instantly applied and is a unique feature of Bletchley Park. Block C was therefore built to a particular specification. The scale of the building is an expression of the mass decryption attack that the machines enabled. The building also had to be sound-proofed, given the noise of the machines, provide accommodation for a number of different types of machines while also allowing enough daylight into the building for the operators and data analysts. The significance of the data and the machinery to Bletchley Park and to the war effort was such that the roof was also hardened in case of aerial attack. Some alterations, such as the addition of ventilators for the machinery, were made in 1943. After the war Bletchley Park became a training facility used by the General Post Office, the Control Commission (responsible for monitoring post-war Germany) and was the home of an Emergency Teacher Training college (for fast-track training of teachers.) Block C became GCHQ’s training school and was altered accordingly with partition walls erected in the previously open-plan spurs to provide lecture and demonstration rooms, a cinema and projector room as well as student accommodation. GCHQ finally left the building in 1987 and it has been redundant since that time.

In 1992 the proposed demolition of some of the Second World War buildings prompted the establishment of the Bletchley Park Trust. The site now opens to the public as a museum and visitor attraction although not all buildings are accessible (and Block C is not at present).


Block C is located on the eastern edge of the Bletchley Park complex, immediately adjacent to, and north of, the new main gate onto the site. It is a spider block and is 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 of the immediate post-war GCHQ occupation. The building is currently mothballed and most windows are therefore boarded up externally although their scale and sloping red tiled cills remain visible externally. However, internal inspection confirms that in the main the windows are 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. These 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 and is 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 the Spur 9 machine bay with a loading bay into Spur 5. Further pedestrian doors have been added and date to the immediate post-war period for example providing access to the ends of the other southern six spurs. There is also a further large entrance with double doors off-centre in the north elevation. Here the flanking panels have been infilled in Stretcher bond and there is also a rain canopy supported on brick piers.

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

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 form of the 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 can be seen where suspended ceilings have not been added (such as in Spur 2). Original internal walls are Stretcher bond using plain flettons, also painted white. Most of the trusses are of steel construction although of differing forms. Internal wooden doors are either of solid form or have glazed lights, all with simple architraves, some with their original Bakelite handles. Post-war additions, where rooms have been created for student accommodation, have borrow-lights above onto the spur corridors. There are also some Bakelite switches and early cable trunking in parts. Cast iron radiators are found in many of the rooms and, as they appear on 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. These spurs were originally subdivided to provide offices, kitchen and ablutions. Spur 2 is perhaps the most original in form of all of the spurs although some modifications were made when converted to GCHQ training accommodation in the 1940s. Spur 1 has additional reinforced concrete trusses supporting the roof level water tower. The post-war conversion of the toilets to bath and shower rooms here 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 also been subdivided for training accommodation although 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 also been subdivided in the post-war period although again the original piers and limited number of original subdivisions of 1942 remain visible. The original spine corridor in the centre of the building, also Spurs 7, 8 and 9 in the north of the building (which were the main machine operation areas) have also been subdivided although again the original piers are visible. The roof of the northern spurs has failed in part allowing water to get into the building such that the suspended ceiling was collapsing at the time of designation (2011).

A more detailed description of the building can be found in Monckton et al (2004, 376-397).


Kathryn Sather & Associates , Bletchley Park Conservation Management Plan, 2011,
Lake, J, Bletchley Park Conservation Management Plan, 2004,
Lake, J, The National and International Value of Bletchley Park: a platform for discussion and its future, 2005,
Monckton, L, et al, Bletchley Park, Wilton Avenue, Bletchley, Milton Keynes MK3 6EB: Historic Building Report. English Heritage Architectural Investigations reports and papers reference B/010/2004., (2004)


This building is listed under the Planning (Listed Buildings and Conservation Areas) Act 1990 as amended for its special architectural or historic interest.

End of official listing

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