Pilot episode filmed on location in Wales.
The front gate is of the Main Gate House, of former St Albans Prison, Victoria Street, St Albans, Hertfordshire.
The communal areas of the prison were filmed in a water tank at Ealing Studios.
External scenes were filmed at various London psychiatric hospitals.
Most of the other interior scenes, such as Fletcher's cell, were filmed at BBC Television Centre.
- First broadcast
- Sunday 1st April 1973 on BBC Two at 8:15pm
- Last new broadcast
- Friday 25th March 1977 on BBC One at 8:30pm
- Last repeat
- Tuesday 6th February 2018 at 12:20am on Yesterday