The skull of the pagan god Dionin used in the movie was constructed by adding sculpted sections to a real cow skull. The original teeth were pulled and replaced with fabricated ones to simulate the serpent look. Two skulls were fabricated for various scenes in the movie.
The mouth of the White Worm (Dionin) seen in the tunnel towards the end of the movie was actually a painted over Volkswagen Beetle hood that bears an uncanny resemblance to the mouth of a snake.
(at around 14 mins) When PC Erny talks on the police radio, the voice on the other end is director Ken Russell.
In a 1999 interview with The AV Club, Hugh Grant recalled that Ken Russell would, following a rather liquid lunch, direct him using language like "Do it how I showed you, you fuckin' cunt!"
The D'Ampton worm is an obvious reference to the Lambton worm, which is a real English legend, dating back at least to the mediaeval period.