ideal isar combination boiler radiators keep getting hot!?

Hi My radiators heat up when they shouldnt especially when running the hot tap. Also the water runs hot and then cold and was told by warm sure that this is because of cold pipes in the ground and nothng can be done. Is this true? I also need to replace batteries in thermostat and dont know if I have to re align it to boiler. Can any one help please. My husband and children are fed up with cold baths and a stuffy house. It is costing a fortune on the gas meter too!
thank you