In September 1940, the Plessey works in Vicarage Lane, Ilford was manufacturing component parts for for Rolls Royce Merlin aero engines. On the night of the 18th, the Luftwaffe did terrible bombing damage to the machine shops. A further attack on 9th October sealed the fate of the factory. Major work would have been needed to get the factory up and running again which would cause unacceptable delays in production. The directors of Plessey looked for an alternative solution.
Before the war, London Underground had extended the Central Line in East London from Leytonstone via Redbridge, Ilford and Wanstead to Gants Hill. The tunnels had been dug and lined but the contractors stopped work before the tracks were installed as finances were diverted to the war effort. Plessey came to an arrangement with London Underground and the empty Central Line tunnels became a factory 13 feet 6 inches wide and five miles long. This secret ‘factory’ was known as Cambridge Park.
2,000 operatives worked here at any one time (2,000 per shift). This 300,000 sq ft production facility, made all manner of technical parts including: wiring harnesses, starter motors, gear levers and field telephones. The shift system ensured that the facility was in operation 24/7. Conditions were dirty and grim, very cold in winter and it was a trial to get so many people and materials in and out every day. The tunnels were however relatively safe from air raids. Former workers have commented that it was like working in a mine.
There was apparently an excellent canteen facility at Redbridge. Additional ventilation shafts were installed to the surface. Extra loading and entry points were added at Danehurst Gardens and Cambridge Park and bicycles were used below ground so that workers didn’t have to walk so far underground to get to their places.
The existence of the factory and its purpose was kept a closely guarded secret throughout the war. The factory closed in 1945 and after 12 months work decommissioning and removing the concrete floor, the tunnels were handed back to London Underground. 12,000 tons of equipment and broken up concrete had to be removed before track could be laid in the the tube tunnels. The former factory reverted to the Central Line and opened for business in December 1947.
Plessey workers further contributed to the war effort by collectively donating a proportion of their wages to a Spitfire Fund. In 1941 they funded the manufacture of Spitfire AD381 which flew from RAF Digby in Leicestershire throughout the war until June 1945. It was ultimately brought down due to a flying accident, not enemy action.