the moment you have seen the gas installation and you are a gas competent person, you are totally responsible for it.
get a copy of 5th edition giusp, corgi still givimg them away.
its frightening to say the least.
imagine youve seen a boiler installation, with inadequate ventilation when you go to price up for a new one. you leave to write up your estimate, 1 hr later somebody dies in that room. its your problem.
ID situations you have no choice but to disconnect, (with the exceptions listed in book)
AR ask for permission to turn off,(hopefully he says no for your sake) label appropriately, get site manager to sign your paperwork passing the responsibility to him. explain the law to him. ring transco.
if he decides to open tap after you leave its his problem!!
thats the easy way. your paperwork is your get out of jail free card.
in a perfect world you would of course fix the problem there and then!!
worlds not perfect.
shaun