The worship of 'demonic heads' was one of the charges in the trial of the Templars, and lies behind the serial's use of the face of Asmodeus. In reality, the 'Templar heads' were merely the supposed relics of early Christian martyrs, such as St Euphemia, which the Templars venerated in typical mediæval fashion. The knights were brutally tortured into confessing that they were something more sinister.
In 1317, the castle of Lindos, used for the exterior of Lavallière's castle, was where Foulques de Villaret, the rogue Grand Master of the Knights Hospitaller, was besieged by his own Order, which had deposed him. De Villaret may have inspired elements in the historical back-story.
Michael J. Bird had first used the character of the psychiatrist Dr Phillimore in the 1971 episode 'To Lay a Ghost' in Out of the Unknown (1965). In this, he was played by Peter Barkworth, but his profession and some of his dialogue strongly suggest that he is the same character played here by Trevor Baxter.