Sorry to hear about your problems with your paint work. Asothers have suggested, bring in trading standards, notify your local press; no manufacturer enjoys adverse publicity. Suggest the paper seek a response from the manager of the showroomregarding your vehicle.
Personally, l would leave the vehicle outside the showroom with an appropriate message placed upon it displaying the virtues (or not) of the manufacturer, and what they are not prepared to undertake.
You have to engage 'the dog with a bone' attitude. do not relent, they will eventuallycome to a favourable agreement with you. It is not in their long term interest to ignore you.
Good luck, do not give in!