Established that it was the flush valve washers! So changed the valve washers on all three toilets. Water usage now back to 3M³!
Must have been quietly leaking, unnoticed, for a long time.
Job done, and no need to change the bottom...
You could have water running in the house somewhere. A WC overflowing (for example) and there by drawing cold water into the house and causing condensation at the main stop tap. Check all WC's, dripping taps etc.
One way to check is if...