the pilot lights but the flame doesn't spread across the burner to the flame sense electrode this happens on both heating and hot water but is a intermittent fault the boiler could work all day no problem then fail
the intermittant fault is the most difficult to find, i would try worcester to see if it is a boiler they still repair, if not then it will have to be the gas board, it could take several changed parts before the fault is diagnosed, and with a fixed price repair these two companies will keep returning till fixed, make sure you try worcester first if you want to follow my advice
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