Inform them their work is pyss poor and the customer is not happy and was asking if there was a special complaints procedure to follow, with regards to contacting their scheme provider over this. That usually does the trick!Trician is a member of NICEIC and I have checked him on their website.
Annoying thing is, I suggested him to the customer based on previous work he did on another job last year.
Well it's difficult to tell via picture but might get away with 12mm basecoat and skim.No Dot and Dab.
But if the area is tiled, there could be allowances for boxes proud for tiles.