I have a Halstead Finest Platinum Combi boiler. The central heating works ok but when turning the hot water on the water heats for about 30 seconds and then goes cold with the reset light turning to Red. I have had 6 call outs by an engineer and nothing has been resolved. The engineer has replaced the following items: Diverter Valve, pump and the central heating sensor but I'm still left with the initial fault. Any ideas what the fault is?