Why is it so expensive to have kitchen fitted, it took him 4 days, and they were not full days.
Not a full answer, but whenever I fitted kitchens in the past I always seemed to go through generous quantities of screws, plugs, brackets, sealants, waste pipework, plumbing components, MFC, edge banding, router cutters, jigsaw blades, circular saw blades, etc, etc. Contrary to normal practice, you can't run router cutters for more than a few cuts, jigsaw blades need to be fresh and sharp, likewise circular saw blades - all of which cost money that comes out pf your day's rate. I used to reckon that my daily outgoings on that alone would be maybe £25 to £30 a day - and that was 15 years ago.
Full service kitchen fitters (i.e the guys who can do plumbing, can core walls, do hob extraction, can move electrical sockets, maybe do a bit of tiling, fit granite/solid surface/laminate/glass/solid wood worktops, etc) should be very skilled individuals who out of necessity carry a massive tool kit, all of which needs to be paid for and maintained. That doesn't come cheap, either. Unfortunately, there are a lot of chancers out there who talk the talk, but can't walk the walk. Frankly, had your man been the real McCoy, £850 for 4 days work would have been cheap