gas engineers have been in court for negligence that killed people, forgetting to solder an elbow in a gas pipe which popped apart months later, flues fitted incorrectly and leaking CO etc, you only have to miss something small but significant to get a charge of negligence doing gas work as the standard required is high.
OP, Price = risk * time, so consider that the engineer could just be hugely busy with low risk work, so no need to take something that could turn into a can of worms. As far as they know, a boiler could have been capped off and decommissioned due to a danger.
I appreciate the implications of the responsibilities involved, I’ve worked on projects that could kill or injure dozens if they go tits up.
Did you actually read my post? They same plumber that drained the system is the one that’s quoted for re starting it, why would he think it was decommissioned because it’s dangerous???