Thanks for your very valuable advice Mr Sonray plumbing.
To rephrase my problem, when the hot water tap is turned on
the tap water gets warm but the radiators get hot, even with CH turned off.
Obviously a changeover valve of some sort allowing the heat
to the CH system as well as hot water.
You are quite correct the water doe's not go to both taps and
radiators as thinking about it the CH is a sealed system, but heat doe's.
As for your advice, "get a plumber", I probably will as it appears to be a bit of a rave to get at this valve but you can bet your life it wont be "Sonray plumbing" with an attitude you appear to have.
Chris Towle, May 2007