Burning Bright by Helen Dunmore.
 
Nadine, a sixteen-year-old runaway new to London, is set up in a decaying Georgian house by her Finnish lover, Kai. Slowly, she begins to suspect that Kai's plans for her have little to do with love. 'Be careful,' warns Enid, the elderly sitting tenant in the house, who knows all about survival and secrets. And when Nadine discovers Kai's true intentions, Enid's warning takes on a terrible and prophetic quality.