If they refuse to pay, you take them to court, but that process is pretty easy (albeit more costly) given you have a Tribunal ruling. I would expect that even the dodgiest builder who might be calling your bluff would pay up before it got to court if they were convinced you were sticking to your guns. The repurcussions if it goes that far are much higher than the tribunal.But if they refuse to honour the ruling and repay the money you can’t do much sadly