Call trading standards and see what they say. If it went to court it wouldn't stand due to poor workmanship and that's clearly visable. It may cost you but with the right legal advise it won't stand. If he won't co-operate cut all contact. Send a simple message telling him your appointed solicitors will be in contact shortly.
This is unacceptable as the point of using a jig and router is to make precision joints and cuts should not be noticed if those cuts are professionally completed. I have fitted many kitchens and never found the laminate of the worktops to affect cuts.
Looks like the fitters jig was either poor quality or it moved while he was routing the cuts out, personally i don't like doing a worktop replacement job because if the existing units are slightly out of of level it can create gaps at the top of the mitre which wont disappear no matter how much you tighten the bolts and forget trying to bridge gaps with colorfil it's a sealer not a filler. Also i have had problems in the past with low quality worktops that are grained especially the black type they tend to leave a visible line on the joints that cant be blended out but if you payed £1000 then the worktops should be high quality with a decent thickness of laminate that is superior to the cheaper worktops with a thin laminate that chips very easily . Most pro fitters will PVA the joins and use a bead of colorfil at the top of the joins just below the laminate so when its bolted together it will ooze out of the join and set and can be cleaned off but my point is if the joins are PVA gluesd like they should be there is no way of taking them apart after the glue has set without causing damage to the joins so it's new worktops i'm afraid