Tunnel Behind The Waterfall by William Corlett
THE MAGICIANS HOUSE William, Mary and Alice spend their school holidays with their Uncle Jack and his girlfriend, Phoebe at the magical Golden House, where they encounter a magician from another time who gives them magical powers and sets thema very important task. . . BOOK 3- THE TUNNEL BEHIND THE WATERFALL It's summertime and the children are back at Golden House, enjoying the sun and relaxing on the banks of the lake, Golden Water. But news of development plans for Golden Water send worries round the house, and the magician warns that they MUST stope the destruction of the magical place. With the magician's evil assistant, Morden, getting closer and closer to time travelling and his magical powers becoming stronger and stronger, the children have a tough job to save the lake and its surroundings.