@David_Jenny0 As you indicate these guests have been a pain all around, it sounds like maybe they are just angling for a refund by claiming there's no hot water. They probably didn't anticipate you coming over to reset the boiler. But if you haven't actually checked yourself to make sure there's hot water coming out, then it's hard to dispute. Maybe they are taking 1 hour showers and expecting that they'll just have constant hot water.
I hear you about no one else even wanting hot water. Here in my part of Mexico it's sweltering hot right now. My water comes out of a big black tank on my roof. I don't even light the boiler from May-October. It's actually warmer than you want out of that tank, for both me and my guests. Altho if a guest wanted super hot water, of course I'd light it for them.