Whilst working for the GPO as an engineer, Fensom continued his education at night school, studying for City and Guilds at East Ham, Borough and Northampton polytechnics.

[4] In 1942, Fensom was sent to work at Dollis Hill Research Station with Tommy Flowers, Sidney Broadhurst and William Chandler.

By the end of the war there were ten functioning Colossi working around the clock helping to decipher the messages of German High Command.

In 1957 he was given charge by Sydney Broadhurst of the system design of a machine called ERNIE, which was to be used to deliver the winning numbers of the prizes for the Premium Bonds.

[11][12] After his retirement, Fensom assisted Tony Sale in the Colossus rebuild project at the National Museum of Computing.