Anyone got ideas re a fault with my combi. It is a Vokera linea 28 which is about 6 yrs old. When I turn on the hot water tap the boiler fires up and works for about 20 sec's it then turns off. The central heating works fine, turn it on and the boiler fires up and works no problems, however, as soon as I then turn on a hot tap it goes off again. I am assuming, after going through the fault finding chart in the manual it could be either the PCB or gas control valve, anyone got ideas on which it could be.