The Black Windmill, 1974..
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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."
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.
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.

