If you sold a product with technical support and restricted access without stating this limitation at point of sale you are in breach of the Sales of Goods act.
Seems you legal knowledge is as good as your gas - no such Act any longer
I know what you mean @newboy but even if he wants to play that all gas equipment is sold on the well advertised understanding that it is fitted by an RGI. If that misses them then it is still in the installation instructions specifically stating that it should be installed by an RGI.
Can't be much clearer unless you don't want to read it.
Jon