I think you just need to treat it as a legal matter and simply look at it as unproven allegations to attempt to extract money of you.
I would be fuming if a landlord accused me of that.
It is essential you have a letter sent immediately to reject the allegation. Keep a copy. You don't need a solicitor to do that first letter, but just keep it basic. Leaving it for a long period before replying seems wrong. Any court will want to see what your initial rely was, although highly unlikely it will go to court.
Do not ever mention later that you know anything about the gas boiler workings! You can later say you never have seen those parts, except in the photos and have been advised by Gas Safe and gas engineers that only a qualified gas engineer can legally remove a gas boiler casing and only normally for servicing.