I remember reading a science fiction book about twenty to twenty-five years ago which I remember very little of. I was also reading lots of Nicholas Fisk books at the same sort of time, but I don't think it was one of his.
The main story mechanism is a series of stones on the floor which the characters can step on and travel through time depending on which footsteps they place on which stones.
All I can really remember is one particular scene in the book where the heroine discovers that she is the one who actually powers these stones, and has to go back to the first time she activated the stones and activate them for her younger self. It is difficult (and I believe painful) for her (although not her earlier self), and her main focus at the time seems to be wondering why her earlier self took so long to do the simple stepping sequence.
I seem to recall the scene taking place in a cave.
I realise that this is a very long shot, but does anyone know what it could be?