From the GasSafe website
When buying a new or replacement gas appliance you should check that it is suitable for its intended location (e.g. in a caravan) and the type of gas that will be used (i.e. LPG or natural gas). It must be installed by a qualified Gas Safe registered engineer.
And from theGas Safety (installation & use) regulations 1998
“gas” means any substance which is or (if it were in a gaseous state) would be gas within the meaning of the Gas Act 1986(1) except that it does not include gas consisting wholly or mainly of hydrogen when used in non-domestic premises;
So you can see that there is no exemption for LPG. However, even if there was, whoever did the installation would be liable in negligence if anyone was injured as a result of a faulty installation and further, this is probably the part that is worrying you, the organisation could potentially be liable along with whoever did the work, on the basis that they instructed an incompetent and unregulated person to do it.
Potentially all the members are on the hook although the reality is that the unincorporated organisation (if that’s what it is) would be sued as an entityalthough all the members would have to pick up the tab.
You are correct to raise the issue and I think it’s irresponsible of whoever has done the work to do so aside the fact that they are committing an offence.
Can I clarify anything for you?
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We can still exchange emails. Best wishes.