My ideal isar combi will not heat the water ?

My ideal isar will not heat the water from the tap unless the bolier is switched to central heating, which at the moment is not too much of a problem as the heating is on quite frequently. I've had a new diverter valve head kit and manifold fitted by a registered plumber but this has not made any difference. This has so far cost me £300 should i continue my quest or am i throwing good money after bad.