Rosalyn Landor (who appears in this film as Mrs Monk) played Sara Latimer in the original 1972 feature film version of this story.
Film critics Mark Kermode & Kim Newman were cast as Mr Karras & Mr Merrin respectively. An homage to the priests in The Exorcist (1973).
Claverton (Elliot Levey) has a speech pattern that doesn't use personal pronouns. This is the same as the character Tom Chance in the 80's sitcom Chance in a Million (1984), who was played by Mr Blunden himself Simon Callow. It's such a distinctive way of speaking it must have been an homage