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.
(at around 14 mins) When PC Erny talks on the police radio, the voice on the other end is director Ken Russell.
The D'Ampton worm is an obvious reference to the Lambton worm, which is a real English legend, dating back at least to the medieval period.
According to the Blu Ray audio commentary with Lisi Russell, Ken's widow, there were supposed to be two additional nude scenes to what was filmed. The first was supposed to be a full frontal view when Sylvia instructs Kevin to stand up in the tub, however actor Chris Pitt got cold feet the day they were supposed to shoot, so he opted to wear swim trunks and was filmed from the waist up. The second was during the sacrifice scene at the end of the film, as Eve Trent was originally intended to be completely naked when tied up over the well. However actress Catherine Oxenberg refused and opted to film the scene in her underwear instead. Lisi noted that Catherine requested they use something in silk, however Ken retorted that this "was not in the budget" so they used Marks and Spencer.
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.