Take stock. The worktops are s***e and should have been mason-mitred with a jig. I don't believe this guy is a kitchen fitter at all. At the end of the day, they are laminate worktops, so not too expensive. The outstanding 25% should go some way to offsetting getting someone else in, so whatever you do don't pay it. You have right on your side, and could go legal, and would probably win, but is the fight worth the hassle. Go to a store, get some new worktop and pay someone decent to fit them.
As far as the other guy goes, you have given him a chance to rectify and he hasn't. Hopefully if you tell him not to come back, there isn't too much other work outstanding. He may shout a bit, but he hasn't a leg to stand on - that is NOT a professional install.