The total number of editions currently known to exist on film or videotape stands at just two (season one's "Ticket To Madrid" and "The Unwitting Courier", along with an incomplete copy of season two's "The Heel of Achilles") out of 25. Additionally, six largely-complete off-air audio editions are known to exist (namely "Return of Evil", "Death in Madeira", "Midnight Over Madeira", "Dead End Dubrovnik", "The Heel of Achilles" and "The Mask of Agamemnon"). The rest of the series is missing presumed wiped.
Associated-Rediffusion were the franchise holders in London and the south east of England for ITV from 1955-1968. When the Franchise was taken over by Thames Television in 1968 almost all the Rediffusion archives were black and white recordings. When the UK started broadcasting in color in September 1969 the majority of black and white telerecordings were deemed of no further value and destroyed resulting in shows like this having missing episodes.
Two novels by David Ray were published by Panther Books in 1966: "All in a Day's Work" and "The End of the Fourth Reich".