Continuity mistake: There is something very strange about the filming of this episode. Scenes such as that of the newly-weds in the hotel grounds were shot during summer, while others such as the truck driver being gunned down involve bare trees and actors' breath visible, so obviously filmed many weeks later. Also, the drive-by shooting at Mr May's (the old chap with the dog) and subsequent arrival of a CI5 squad car arriving are two scenes that were filmed months apart: in the latter the trees look noticeably barer than when the villains' car roared past.
Trivia: This, the first ever episode of the series, was first broadcast on ITV on December 30th 1977. It was repeated on ITV in 1979. Some time after this, a cut was made, which has endured on all subsequent UK TV transmissions: Just prior to the title sequence, we see Nesbitt hand Susan some drugs. In the original broadcast we then see Susan tightening her belt around her arm. Using Nesbitt's cigarette lighter to sterilise a needle, she then "shoots up" (injects the drugs in her arm). The excision may have come about because of a tightening up of censorship rules by the Independent Broadcasting Authority about the explicit use of illegal drugs on TV. However, the missing segment has been restored for the 2002 DVD release.
Trivia: "Private Madness Public Danger" was the first ever episode of the Professionals to be broadcast. However, it was the sixth episode to be made, and was originally to have been the SECOND episode to be broadcast. Throughout the entire six year life of the series (December 1977 to February 1983) the order the episodes were made bore little resemblance to when they were first shown. Some episodes were left "on the shelf" for up to two years before being shown.
Trivia: According to the credits, this episode was written by a "James McAteer". However,"James McAteer" is actually a pseudonym for well-known TV scriptwriter P J Hammond (who, amongst many other things, created and wrote the strange sci-fi series "Sapphire and Steel"). Apparently Hammond did not care for certain "alterations" made to his script (rewrites by the Series' creator Brian Clemens) and asked that his name be removed from it.