What's the mark down the drivers door? Is it a reflection or a graze?
I'm afraid I would not have agreed any money based upon a back street garage. I would have taken it to the main dealer to get sorted out as he is at fault and should pay the full cost, if not put it through the insurance, you won't loose NCB cos it's his fault.
I had to adjust my bumper a few years ago when mine got caught on the ladders in the garage, as far as I recall it was quite easy.