I asked the bloke wot does my van repairs to strip and clean jets on the VERY old Carver Cascade water heater in it and he laughed at the idea that anyone would be able to get the screws that hold it together out. Old and manky is good sometimes...

The thing that puzzles me is that presumably the control panel for your Carver was busily telling you that it was not lighting, so you might have known something was wrong...