Glowworm Ultimate 40FF - main burner keeps going out and relighting?

The pilot lights, its continues to spark then the main burner lights then the main burner goes out after a few seconds but the pilot stays lit then the main burner relights again and the cycle continues over and over. This happens whether its trying to heat the hot water, central heating or both. So far, a corgi engineer has replaced the air pressure switch, pilot assembly, fan, gas valve and PCB but the problem continues. What else could it be?