I'm not nit picking, but (
)
A) Porridge was set in HMP Slade, a fictional prison in Cumbria.
B) The main gate of "Slade Prison" was filmed at The Gate House, St Albans, Hertfordshire. This was was originally built as the town jail, back in the Victorian era. It was being used as a council works depot at the time Porridge was filmed.
C) The "association area", walkways and cell exteriors were filmed using a set built in a large metal tank at Elstree Studios. The tank had previously been used mainly for filming underwater scenes for British films.
Exterior scenes were filmed at various psychiatric hospitals in and around London.
D) Yes to Chelmsford and Maidstone prisons.