William Corlett 
Bridge In The Clouds [EPUB ebook] 

It is deep autumn and William, Mary and Alice Constant have returned to Golden House for the half-term holidays. At once they are thrown into the most crucial and desperate adventure of all. Matthew Morden is on the point of discovering for himself the art of time travel and his evil influence is permeating the whole valley. Now the future of the valley depends on the children reaching the Magician in his own time. The time has come for a final confrontation; the last great battle is about to be fought.