Prompted only by the horror of a trebled water bill, I tracked the source down to a Geberit dual flush valve silicon/rubber seal washer that had deteriorated and become covered with "warts". (Not an uncommon problem! ) Isolating the WC, noting the dropped cistern water level later, and washer replacement followed by a dye test is the cure, but the damage had been done.
How could I have been alerted earlier? The continuous flow was into the WC pan and not sufficient to be visible.
How could I have been alerted earlier? The continuous flow was into the WC pan and not sufficient to be visible.