The Difficult Job of Keeping Time — Dyan Sheldon

Trish is scared of her mother's rages while timid Kiki is scared of the teen gang who terrorize the inner-city estate where they live. In their different ways, they both feel helpless and alone. But then they meet the mysterious Betty Friedman, and old lady with a secret to protect. Before they know it, Trish and Kiki are plunged into a time-travelling quest to save a historic church - and the buried treasure beneath it. Going undercover as servants in Victorian London, their cunning and bravery are put to the test, and they learn that they are more courageous than they could ever have imagined...