Then it's a mystery...
The hot water storage tank - the one with the immersion heater - will be getting it's replacement water from somewhere and there will need to be some form of control on the flow into it. Either there's a tank in the loft or, there will be a lid on the hot water tank in the airing cupboard. This is most unusual but as a child in the 1950s there was a similar arrangement in my bedroom!
Have you checked the toilet cisterns? Have you run the hot tap for a few minutes? Did the overflow stop? Might the overflow be coming from an adjoining property?
Look at you hot water tank - does it have a lid? Is there a fat pipe low down on the bottom rim of the tank running up to the ceiling? If so, this may be the feed from the header tank above.
You have exhausted my ability to offer any more suggestions - my apologies.
Continuing good luck...
P
September 2009