Why would they not? You have a cert of the works tested from the gas installer, it's on his head if anything goes wrong as he was the last one to test the appliance and pipework. Once he signs that bit of paper, everything is on him, no? No different from him getting an apprentice to do it and then he checks and signs off.
That's how I see it anyway. Tbh it's been years since I touched any gas, but that used to be the principle then anyhoo.
Agreed - but I was referring to if he did it without a GS man at all (and no sign off certificate)