Dr Who: Journey into Time
In 1970, The Doctor Who franchise appeared to be in rude health. Jon Pertwee’s new Doctor had arrived on TV, the first to appear on the small screen in colour. at the cinema, the Dr Who films jointly produced by Amicus (under their Aaru subsidiary) and Hammer Films had been evolving and gaining a loyal following for Peter Cushing’s eccentric Dr Who.
Before the fifth film, Dr Who and the Yeti Invasion of London, was released work had already begun on preparing the sixth instalment in the series. Armed with [a] list of possibilities, the two producers began to cement their plans [for a sixth movie]. Those, however, were derailed when Peter Cushing was forced to cancel all work commitments for personal reasons. The actor would not work for a year.
[Michael Carreras] … proposed a radio series for the BBC featuring Peter Cushing as Dr Who. That could be recorded quickly when Cushing was ready to return to work, and would bridge the gap with the public until they could get a new Dr Who film into theatres…”
– Extract from From Shepperton to Skaro: Dr. Who on the Big Screen by Michael Loggins.