this happens more than you think,my old chap was a top plumber and taught me a lot,i,ve lost count of the number of botched plumbing jobs (gas) that i,ve seen one local bloke asked me to look at his fire as it kept going out and would only relight after the door to living room was left open,did a smoke test on flue and it was blocked, checked on roof and found roofer had removed "redundant " chimney pot and capped flue with a concrete slab! another was a well known local plumber/gas fitter who was removing a gas heater at a friends house,I watched him cut the gas pipe to cap it then realise he had no solder,so got him some out of my van,when he finished and packed his gear away I asked about a drop test which he hadnt done,he then said he had not done one before starting so if there was a leak he had no idea if it was there already or he had caused it.Then people wonder why I get wound up when they say "you have to be qualified to work on gas"from what I,ve seen I,m safer than some of these so called professionals.