When this happened to mine it was a split diaphragm in the divertor valve which is what switches the water to be heated instead of the radiators whenever the tap is opened. You can replace the diaphragm with a service kit.
Have a look to see if the valve moves up and down when you open the hot tap - it should press a block of 3 microswitches. If not its likely to have split.
Adam, October 2005
