I had the same thing. My problem was the motorised valve, which directs the hot water into the radiator circuit, was not opening far enough. It did open, but there's a switch in the valve which is only made when the valve is fully open. The boiler will not kick in for heating only unless the switch is made. These valves usually have a manual lever on the outside to force the valve open. Try moving the valve this way. it may take a few goes if the valve is sticking, and see if the switch makes.
Steve DIY, March 2010
