Send them a letter giving them 14 days to pay then take it to small claims court. I never let anyone take the pi$$ whether it is £85, £850 or £8500. You did the work you want payment end of story IMO.
Alternatively especially with that type of business, I would also be bloody minded enough to sit or stand outside his front door with a placard saying this rip off merchant got me to do work and doesn’t pay his debts. If I were him I would give you double to get rid of you and avoid the embarrassment