I'd be asking what they meant by a conversion pack.
You believed someone would come and fit a boiler for free and didn't realise the plumber's wages came out of the mark-up made on the boiler, yet you claim to be some form of solicitor. You are also missing the costs of parts, filters, controls, water treatment, etc., none of which are given to plumbers for free. This is getting weird. I'm also out on this part of the question.
What IS a good question is who would be best to get a report from. Not sure if Building Control would be a good start or what they might be willing/able to do at this stage?
Seriously though, I expect every plumber in the world will say they would have done it differently, however well or badly the work was done.
I'm in no way a lawyer. I just work for a small business in a bad industry and have ended up doing a lot of bog standard claims over the years. Going to court on a small claim isn't an issue for me. I know that may seem arrogant, but it is part of my job and is kind of second nature to me. I actually fix circuit boards and printers most of the time.
In 2018, claiming free fitting, then charging for a conversion pack that doesn't really exist is completely prohibited behaviour when dealing with consumers. I was just hoping the work was also prohibited by some obscure regulation. I will call building control on Monday, I think you are right suggesting them. They may refer to gas safe who sign it off by proxy for them maybe?
Thanks for the help.