It does look as though the main cause of the rot that spread throughout our bathroom floor came from the toilet cistern, which is boxed in and seemed to be forever sweating litres upon gallons of water. After fitting the new floor, we lifted the lid to find condensation was still running and forming around the outside. Insulation is a popular solution to this, with the internet throwing up the suggest to lag around the inside with either an old gym or exercise mat or, good old expanding foam.
All along, I’ve felt as though ventilation (drilling a hole through the exterior wall and fitting a vent) would fix this. My dad’s solution was to throw a towel over the cistern and to ‘insulate’ it that way… Although the situation eased at first, it didn’t go away.