I've just spoken with the plumber and asked him all the above. He says he's checked everything and that it must be a leak inside a wall in one of the bathrooms to the side of the house as when the separate valve to the kitchen is turned off, there's no problem. I did point out none of the walls were damp and I'd've thought the tiles would be dropping off by now, he replied the water was probably draining into the ground. I believe him, he's one of the rare as hen's teeth good workers. So must assume the 'eaten by rats' or just plain deterioration theory. I also asked him about a 'Leak Detector' - a sonar device, he knows about them but says they're very difficult to find here and frankly, prohibitively expensive. So, he's installing the switch as I type. Ah well. Will just have to turn the valve to the bathrooms on when necessary as it would indeed be a PITA to have him rip all the tiling off, replace them, and then still hear the bloody thing turning itself off and on again. Thanks everyone for taking the time to reply.