I asked my friend, she says that the old house has been built on land that was level, but over the years it has sunk slightly down deeper into the ground, and they do have a stream running close to the house. So as the house has sunk slightly lower than its clay surround it has formed a slight hollow which gathers up all the excess water running off the land around it, and the stream probably doesn't help there... As she encouraged its route she is keeping a bit quiet on that issue! They are going to have to have a tile-lined French-style storm-drain trench dug all around the outside walls so that the water will drain off into the back garden, not all soak into the ground.