False pretences - Margaret Yorke

When Isabel's god-daughter, Emily, turns up after years of no contact and in need of help, she feels duty-bound to take her under her wing. To her surprise, Emily is determined to be independent and takes a job as nanny. Emily's charge turns out to be the illegitimate child of the nave daughter of a well-to-do couple and a ne'er-do-well conman who disappeared before the baby was born. And now he is back, intent on exploiting his parental status in return for cash.

Before she knows it, Emily is caught up in his botched attempts at blackmail, trying desperately to protect her charge from harm, while also shielding Isabel from becoming entangled in the drama. But when events beyond her control force her to act instinctively, with horrendous effect, all their lives are put terribly at risk.