The amount of the ransom money in the source novel was $500,000. But in this movie, the ransom was represented in the form of high-grade uncut diamonds, worth £517,057.
In his memoirs, Joss Ackland wrote that he was working on three other movies, as well as playing Wray in this movie.
The marquee of the theater, in which Tarrant and his wife have their appointment, advertises "Battle of Britain (1969)," which also starred Michael Caine.