This is a long shot as I don't remember much about it. I think it was for older children, and the action was based around a cottage that the protagonists believed belonged to a witch. At the end of the book the thatch of the cottage caught fire, and someone found some small bottles in the thatch - one of them contained human blood. That's all I can remember, and some of it might not be right... Any suggestions gratefully recieved as it's annoying me!