Given that the episode opens with a scene in which Cowley describes the purpose of CI5 to a group of new recruits (and hence to the viewer), it is clear that this episode was intended to be shown as the first one. However for some reason ITV scheduled it as the third one at the initial showing and this order has persisted for all future showings.
Gordon Jackson was unavailable for the first three weeks due to commitments on The Medusa Touch (1978), so the shooting schedule was rearranged to accomodate this. The siege scenes had to be split over several days spanning two weeks as a result.
This episode was the first to be filmed. Some scenes were filmed with Anthony Andrews as Bodie and were later re-shot with Lewis Collins playing the part.
Cowley impersonates the Home Secretary at the police station. The Home Secretary at the time was the Right Honourable Merlyn Rees, in office 1976-1979.