The plaster is loose as well. Hoping it's a small patch I can fix after removing loose plaster. It's not just above the cistern, it's behind it as well. (Waiting to get cistern removed.) The plaster is bone dry, no damp or crumbling. But it can't be a coinidence it's around the cistern, can it?
As photo shows, I removed loose tiles, and the wall is cracked.
A builder said strip all walls back. I'm going to put the tiles back and hope for the best. This is above the cistern. Any suggestions?