Can anyone help I have had 2 engineers to look at my boiler but it's still not right, it has an intermittent fault that keeps locking out only when using DHW it’s fine if running the heating but when I turn on the hot water after about 1 minute it lockout, I reset boiler heating work ok again but I get the same result if I run the water again, so far I the engineers have had put a new PCB board in have cleaned the venturi, and flow sensor valve