Mystery Man question

Around 2008-9, I bought and read a book about a lady who moved into a village, possibly to grieve for the loss of her husband. She receives occasional visit from friends, but becomes increasingly distrubed when she feels she is being watched by a man, for example, when she goes for a walk she sees him at the top of a distant field. The situation becomes worse over the course of the book,until the 'man' attacks her. It turns out that an elderly couple who have lived in the village for years had a daughter with special needs who was disturbed by the arrival of the lady into the village and eventually attacks her. If I remember right, the woman ends up killing the daughter in self defence, when the truth comes out. Does anyone know the title and#/or author of this book please? It's driving me mad.