Mr RIU took Hank for a service and had to have new brakes. Quite annoying when it was purchased from a main dealer only a year ago and has dome limited mileage since. Whilst he was out, unusually I went to the bathroom and spotted a yellow patch on the ceiling.
I turned the water off at the mains, located a step ladder and torch and heaved myself into the loft space to establish the cause. The cover on the water tank had fallen inside and jammed the ballcock/blocked the overflow.
The chipboard that it and the hot water tank stand on has taken in a lot of water and the plumber who came later advises replacement.
More expensive. It could have been much worse had I not spotted it when I did. The bathroom ceiling can be sealed and painted.