In order to have the damage repaired you will need the garage to accept responsibility for the damage, or if necessary take them to small claims court. We would suggest writing a letter of complaint by recorded post to the dealership, providing any evidence you have to prove that the damage was caused while your car was in their care and asking them to repair the damage at their cost. If they refuse, you can try contacting the dealer's insurance company, trade association or complain to the manufacturer directly.