Hi Bren,
The fault is probably the diaphragm in the diverter valve, this is inside the round brass unit on the end of the diverter with 2 6mm chrome pipes going into it. You have to isolate the mains water under the boiler or house stop tap and turn on a hot tap which should stop running. The unit can then be removed by disconnecting the 2 chrome pipes and removing the 2 screws which hold the unit onto the diverter valve. Once off undo the 8 screws and split the unit to reveal the rubber diaphragm which hopefully will be damaged. These can be purchased from most heating spares suppliers.
Ian
September 2008
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