This is all something I learnt bout 15 years ago, I had been barred from about 5 bodyshops due to poor workmanship.
The difficulty you have is that it only has to be repaired to a reasonable standard, that could go as far as the paint not matching etc.
The only anwer to this is find yourself a decent bodyshop and use them everytime regarless of how hard your insurer pushes you.
When I crashed my Elise I had just had it painted in a 7 way flip gosting £6K, it had a special twist to it to make the effect even better. Anyway the insurers tried to ge tme to send it 150 miles awy to their recomended repairer who deals in standox paint (the manufactuer of the paint) Thye could not guarantee a match but if they can not match it (which they wouldn't due to the extra twist) they will just repaint the car how standox recomended and thus give me a diffeent colour.
The argument went on for 10 months!!!
I eventually got the car repaired where I wanted (It did change colour though.....to a 21 way flip)
The simple answer is Do not use an insurance approved repairer, they are working for the insurer not you (they rely on the insurer for the majority of their business) and don't use a manufacturer aproved repairer s they are not bothered as they are guranteed a flow of work from people who think they have to use them for warrenty etc.
Get an independant specialist/customer painter who you know works to YOUR expctations
Back to your difficulties, forget asking the insurer for help, get your own report (search under assessor on yell.com) it will cost you about £80-£100, send this to your insurer, advise them you have a contact with them to repair your car, they have failed and you look to them to retify the car by doing X, Y an Z etc. Make sure you state it is a formal complaint as this forcesit through the complaints proceedure, rather than being classed as a notice of disatisfaction