I’d definitely recommend legal advice, and I think I’d approach in this order…
1. Put in writing to him that you have concerns and why they are and that you’d like them rectified
2. Arrange for another reputable spark to inspect and report on the install as it stands, take loads of pictures, if
@UNG has offered take him up on that offer
3. Now go back to the original installer with the report and give him in writing the chance to rectify, personally I think at this stage he will break contact once he realises an actual electrician has been involved.
4 give him a week or two and keep logging your contact attempts
5. Now you can contact solicitors, his scheme, trading standards and building control
I’ve recently been involved in putting right works by a cowboy builder who fleeced the customer for £30k for some absolutely shocking works, they actually got him to court and he was prosecuted, he ended up with an 18 month suspended sentence for issuing an EIC for the electrical works despite not being an electrician!