It’s driving me mad! It was quite a popular book, up to about 20 years ago, maybe 10-20 years ago? A mystery written for adults. A child goes missing (I believe a girl) and the mum grieves. One day a child turns up who is almost identical turns up except years have passed so it can’t be her. I’m sure it ends with the school playground being dug up and a body found under a new building or something. Does it ring any bells?