Moondial, by Helen Cresswell
Lux et Umbra Vicissim, sed semper Amor - Light and Shadow by turns, but always Love.
This book was turned into a children's tv series in the 80's, which is where I, and probably a lot of other people, first came across it. Got this copy for 20p, which has to be one of the bargains of the year.
Minty (Araminta Cane) is staying with her Aunt in the village of Belton, while her mother is in a coma following a car crash. Drawn to the sundial - or moondial - in the grounds of the big house next door, she discovers it is a link through time, meeting up the kitchen boy Tom and the persecuted and mysterious Sarah in two different periods of history. Threatened by the mysterious Miss Raven, it's a race against time to save the children and pull her mother back from 'moontime'.
I remembered quite a bit of this from the tv series, but what I'd forgotten and really wish I'd found out in time was that Jacqueline Pearce (Servalan from Blakes 7) played Miss Raven. Might have to re-read her bits and imagine them differently.
A recommended, nostalgic, spooky, beautifully descriptive read.