False Pretences — Margaret Yorke

When her goddaughter is arrested during an environmentalist protest, Isabel Vernon is startled to discover that the fair-haired child of her memory has become an overweight skinhead activist and that Isabel herself is now regarded as Emily Frost's next of kin.
Emily, released on bail to the Vernons at their home in the London suburb of Fordswick, meets a middle-aged drifter who encourages her to take a babysitting job at the home of a local family. Emily forms an instant attachment to Rowena, the four-year-old girl in her charge and, missing her campaigning friends, is eager to make a success of her new position.
Emily's unconventional presence in the Vernons' house proves troubling, and is deepening the profound tensions within Isabel's marriage. Meanwhile, the arrival of someone else in Fordswick threatens the safety of Emily and Rowena, as well as the security of many of the town's long-buried secrets.