Chatting to a neighbour (who shall remain anonymous), I discover that her house is considerably older than she thought it was when she bought it. She knew it was old - maybe 18th century or so. But having looked it over thoroughly and burrowed through some of the paperwork, she thinks that parts of it are 15th century, or possibly even earlier.
So, she checked her insurance. And found that insuring very, very old houses is really rather expensive and difficult. It seems likely that at least the last two owners of the place have been a little discreet about its age, at least in public. She plans to follow their example.