Rattlingly good children's novel by Alan Garner. Cadellin (pronounced Cadethlin) is a wizard. He guards Fundindelve, a cave set into Alderly Edge and home to sleeping knights and their horses, ready, when England needs them, to awaken and march into battle. The magic which controls Fundindelve is sealed inside a stone called Firefrost, which Cadellin manages to lose.
Visiting Cheshire are Colin and Susan. They are attacked on Alderly Edge by a bunch of svarts (nasty goblin type creatures) and get rescued by the wizard who explains his sad plight. The children, full of the story they've heard, return home. Unbeknown to any of them, 'The Bridestone', a family heirloom, which is on Susan's bracelet, is in fact, Firefrost. It was the stone's presence which alerted the svarts to the children. Cadellin has managed to lose the stone twice.
The stone is then stolen by a shape changer called The Morrigan, and the retaking of it, and the journey to reunite it with Cadellin take up the main body of the book. It's enjoyable and it's easy-going, certainly, but it's also massively frightening at times.
Interestingly enough the tale of a wizard and the knights under the hill is not Garner's invention but is a 'real' legend about Alderley Edge which Garner has adapted for the story. The pub called The Wizard, referred to many times in the book, exists, beneath The Edge, although it is now a rather fine restaurant.