Regarding billing, the actors who play Miss Higgins, Cyril, and Reggie have been on the main cast list since the Series 9 Christmas special.
Matthew has been on the main cast list ever since his first episode.
By main cast list, I mean their names appear in the opening credits.
I have a question regarding Christopher who was Trixie's love interest: Was he always intended
to be a short term character or did Helen George's pregnancy force them to eliminate him?