The Chaos Quest by Gill Arbuthnott
The sequel to The Chaos Clock It is eighteen months after the events of The Chaos Clock, and Kate and David are now at secondary school in Edinburgh. David is struggling to come to terms with his new stepmother, and Kate is being expected to take more responsibility for her younger brother. But time never stands still for long. They soon become involved in a race to prevent the Lords of Chaos from tricking Erda, the Stardreamer, into losing her power. Even with help from Morgan the Hunter, can they prevent the barriers between times being blown away forever?