Realizing there are probably differences in plumbing between the U.K. and here in the U.S., there is no interconnection between hot and cold water systems. My guess is that someone has worked on the system and hooked it up backwards. In our system, there's really no other explanation. Here, the main water system enters the house in one supply pipe, enters a "T", one side of which goes to the hot water heater and the other side serves all of the various outlets, toilets, etc. The hot water comes only from the heater and has no access to the cold water pipes...