Michael Gothard had been cast as a Russian agent, Kodai in the popular TV series, "The Professionals."
According to "The Professionals" fansite,
CI5, filming took place between 23 April and 4 May 1979, though the episode was not shown until 10 November.
Among locations used were:
Friern Barnet Lane, North Finchley, London, (Kodai's and Radouk's house)
Frithwood Avenue, Northwood, Middlesex, (safe house)
White Waltham Airfield, Maidenhead, Berkshire, (RAF Manley airfield).
The stunt arranger was a man Michael had worked with on “Arthur of the Britons” - Peter Brayham.