how to fix my vaillant 242 e-h combi(fires up when not req)?

Does the central heating come on properly if you switch the heating switch on? Is the first bit of hot water unusually hot?
The boiler has hot water priority and if the water valve is sticking on it won't allow the heating to respond and will keep trying to bring the boiler on for hot water. You may also find that the pump never shuts down. Need to know more..

T.H.S
May 2007