The titles/credit sequence was shot from Alpha Tower, looking down onto Paradise Circus in Birmingham city centre.
There was a fourth guard called Mr Smith who was a corpse sitting in the office. He had died three years before and the rest of the guards were dividing up his pay packet.
Both actors Robert Lindsay (Carter) and David Threfall (Bell) are in the 1983 film King Lear with Lindsay playing the role of Edmund and Threful playing Edgar. The Nightingales episode 'King Lear' was most likely a nod to this.