Just watched this again, and it hasn't improved, though slightly better than Up Pompeii imo
Up The Chastity Belt Page 2
It starts off really well then tails off a bit for me when it becomes less enclosed, as he goes off on his crusade. Still full of good rompy lines from Galton & Simpson most of the way. Pompeii! though for me is the best of the films, being like an extended single set sitcom with it all bubbling up in a pressure cooker environment.
Most sitcom films go the other way and try to be too filmic, and lose something doing it. Up Pompeii! retained most of what it was, for me.