Trying to remember a book I read at around age 12ish in the 80s. It has been haunting me for ages and I have googled all sorts of things with no success.
The basic plot was a girl who went on holiday with her annoying friend and annoying friend's mother. (I think was written in 3rd person from main girl's POV) They meet a mysterious other girl who was around the same age and who somehow joins them on their holiday with some scary results. I'm fairly sure it was a British book but can't remember if the holiday was in the Uk or overseas. The mysterious girl may well have turned out to be a ghost or time traveller of some kind, I don't remember. I remember one bit where they are all going round some kind of ancient/ old building (which was probably Significant to the plot) and the Mother says something along the lines of " Slow down girls, the passage is narrow and I am wide" and this makes the main protagonist feel guilty about how much the mum is putting herself out to make the holiday nice. I kind of think the annoying daughter was called Gwendolyn or similar and the mum is Mrs Oliver, but it is entirely possible I'm just mixing the characters up with the Malory Towers Gwendolyn and Agatha Christie's Mrs Oliver - they were kind of similar to those iyswim.
That's a very random description, but if it rings any bells with anyone I'd love to read it again.