I have had an itermitent fault the boiler would fire up either DHW or CH then after an hour or so if trying to fire up after going on stand by it would not light and lock out in either DHW or CH this has been going on for a few months I have had it serviced and looked at a few times but now it will not light even 1st off it will go through sequence and some time flame will light up and go straight out some times just lock out with out flame