Having fixed various pressure and sludge-related issues I'm still have trouble with an annoying intermittent fault. Most of the time the domestic hot water works fine, but sometimes it doesn't - the burners just don't light apparently. I say "apparently" because I don't live there, and I just get rather vague reports that it's not working properly, and I'm unable to replicate the fault when I show up.
Anyway, suggestions as to what could cause an intermittent DHW fault like that?