 The BBC production team had planned to film Porridge in a real  prison however before filming began the Home Office refused. The BBC  improvised. They used the Main Gate House, of former St Albans  Prison, Victoria Street, St Albans, Hertfordshire as the main gate  of "Slade Prison". Slade Prisons metal walkways and communal area  were filmed in a large metal tank at Elstree Studios. The huge tank  was previously used for underwater filming however the BBC converted  it into the two floor prison seen in the programme. External  scenes at "Slade" (e.g. the rooftop strike, exercise ground) were  filmed at various London psychiatric hospitals. Most other interior  scenes (e.g. Fletchers cell) were filmed at BBC Studios.