The helicopter explosion was taken from From Russia with Love (1963). It had previously been used in The Enemy of the World.
This episode was watched by 8.1 million viewers on its original transmission.
The colour master videotape of this episode was wiped by the BBC in the 1970s and the BBC only held a 16mm black and white telerecording. A poor quality colour NTSC version taped on a home recorder also existed. The episode remained only in black in white until the 1990s, when the colour signal from the NTSC version and the black and white telerecording were used to create a new colour master on D3 digital tape.
The Royal Air Force aircraft seen is this episode is an English Electric Lightning operated by 111(F) Squadron, then based at RAF Wattisham in Suffolk. The squadron crest is clearly seen on the tail of the aircraft. This squadron was also used in another BBC TV sci-fi series, 'Invasion: Earth' (1998), when it was equipped with the Panavia Tornado F3 long range interceptor.