In his memoirs, Joss Ackland wrote that he was working on three other movies, as well as playing Wray in this movie.
The machine guns used by Major John Tarrant (Sir Michael Caine) and McKee (John Vernon) at the end are Ingram MAC-10s, probably 9mm. The guns were known for their high rate of fire, making them popular for movie work in the early 1970s.
The marquee of the theater, in which Tarrant and his wife have their appointment, advertises Battle of Britain (1969), which also starred Sir Michael Caine.
This movie was released one year after its source novel, "Seven Days to a Killing" by Clive Egleton, was published.
The windmills seen in the film, are "The Clayton Windmills". Wikipedia states that they are "known locally as Jack and Jill, (and) stand on the South Downs above the village of Clayton, West Sussex, England. They comprise a post mill and a tower mill, and the roundhouse of a former post mill. All three are Grade II listed buildings."
Lew Wasserman, Director Don Siegel's boss, wanted Edward Fox for the role of John Tarrant.
The amount of the ransom money in the source novel was five hundred thousand dollars. But in this movie, the ransom was represented in the form of high-grade uncut diamonds, worth 517,057 British pounds sterling.
Generally speaking, this movie is looked upon unfavorably in comparison with The Ipcress File (1965), also with Sir Michael Caine.
Donald Pleasence played a government section boss in Innocent Bystanders (1972), a low budget spy movie.
John Vernon appeared in other Don Siegel movies. Amongst them were Dirty Harry (1971) and Charley Varrick (1973).
The soundtrack for this was done by Roy Budd, who, also in the same year, scored another Sir Michael Caine movie, The Destructors (1974).
First of two movies that Sir Michael Caine made with Producers David Brown and Richard D. Zanuck. The second was The Island (1980).
The opening credits had the words of its billings formed out of characters that were children's lettered playing blocks.
John Rhys-Davies (Fake Military Policeman) makes a brief appearance at the beginning of this movie. He supported Harrison Ford in two "Indiana Jones" movies.
In one scene, Cedric Harper (Donald Pleasence) declines the offer of an alcoholic drink, saying he doesn't enjoy it. In real-life, Donald Pleasence was consuming more than his fair share of alcohol, according to his stepdaughter.