BG do sub contract to plumbers and electrical contractors in the area local to customers houses. The legal argument is "fitness for purpose" and if the leak did not happen until 5 years after the item was fitted any warranty from the installer or manufactuer has expired.
BG have a duty to rectify the leak under the contract but not to rectify design problems.
Specific clauses will be written in to the BG contract to protect BG from design problems.
I have recently had 8 months of trying to get my bolier fixed with BG finally replacing the right part.
However back to your question, BG will need to clarify the specific exclusion in writing - Citizens advice will help you progress your claim if they don't comply.
Best of luck