Given the crappy workmanship, Small Claims might well be worth a go to recover cost of materials he's wrecked and the money he's charged you to do it. Only costs about £60 and a few hours, at worst it'll irritate the fitter, at best you'll get a judgement against him (available for anyone else to see).
Don't wait for legal process before getting it done properly though, keep all photos, emails, text conversations, invoices, make dated notes of conversations (as best you can recall, resist the temptation to embellish).
Good luck