The fifth episode, "Happening", features William Gaunt's character emerging unscathed from a fire. It just so happened that a fire was raging at a building nearby to the Borehamwood studios were "The Champions" was being shot. A camera crew was dispatched to film the real conflagration, with the aim of building a story up around it so that the footage could be used. The same fire footage would resurface a year later in an episode of "Randall and Hopkirk (Deceased)".
Bomb control room wall mounted speakers were previously used as receivers/speakers in The Prisoner (No6 smashes one to pieces in episode 1) and appear again in Nutcracker.