The prison exterior in the title sequence is the old St Albans prison gatehouse and HM Prison Maidstone, which was also featured in the BBC comedy series "Birds of a Feather" (HMP Slade is referred to in "Birds of a Feather" when the main protagonists' husbands are imprisoned there after reoffending in Series 7). The interior shots of doors being locked were filmed in Shepherds Bush police station – the BBC had a good relationship with officers there. In the episode "Pardon Me" Fletcher speaks to Blanco (David Jason) in the prison gardens: this was filmed in the grounds of an old brewery outside Baldock on the A505 to Royston. The barred windows approximated a prison. The building has since been demolished.