My radiators work fine until I switch the hot water on (via the programmer in the airing cupboard housing the insulated h.w. tank). The hot water starts ok but the rads stop. After an hour or two I switch off the h.w. and leave the programmer on C.H. and have to wait two or three hours before the boiler fires up again. Seems I can have EITHER h.w. or C.H. but not both at the same time. Why does it take a few hours to fire up for c.h. after switching off h.w.? Any ideas ... anybody please?