Not sure why everyone's saying he's paid too much. £15k is pretty cheap for a job like that (if done right of course). This is 2023 not 2015.
I really feel for you though mate. Had a nightmare myself about a year ago. Things are still in motion so unable to make a post about it yet, but when I do....what a post it will be!
Have to say though, there is some terrible advice in the first few pages. If you ever decided to get the courts/solicitors/police involved, well....good luck is all I'll say! Think someone did actually say it takes years to get things like this sorted (if ever), they are right!
anyway, we had a much smaller job done recently and that was £10k so don't think you got ripped off price wise.