...and generally trust nobody.
We have a gas heater at work fitted by the heater seller's fitters of choice. I didn't get a good feeling from these guys when they were installing.
Recently it refused to light, so phoned the seller who in turn asked me to phone the fitters. 'Oh yeah', he said, 'its common for those to need a [some regulator or summat] and its over a year old so it will need a service etc, about £250 mate, when do you want me to come.' I told him i'd get back him and something about the way he was mad keen to pin me to a booking made me tell him to leave it.
I phoned the seller again, see if they could recommend a different engineer, fortunately this time I got someone competent, he had me open a paned, unscrew a little cover and push a switch, heater fixed.
Bastards.