Problem with Potterton Suprima 80L?

Potterton suprima 80l, h/w is fine as is c/h as long as BOTH are on. Once h/w reaches temperature and clicks off c/h is OK for about another 15 minutes then boiler only fires and runs for approx 15 seconds then shuts down cutting out pilot light. New PCB fitted last week, divertor valve checked and operating OK, both thermostats showing 240 volts at boiler when activated and pump running smoothly. Wiring also checked and is exactly as per the Potterton diagram using PWB1, EP3002, MPE322, PTT2 and PRT2 with Grundfos pump.

Can anyone give me any clue as to what is causing this problem?