Once you have a taken ownership of the car and driven it away, you have accepted it in its cosmetic condition, for all they know you could have poped into sainsburys on your way home and someone dinged the door. Imagine if you were the dealer trying to run a business and how many times this happens, not saying their out to rob you but you should have checked over the car. I always take new pics of my Zed all over before it leaves my hands for any work and check it over before I pay/drive it off as thats your only real chance of any comeback if anything happens.
If I was you I would go back and be very polite and hopefully they will repair it for you. Its just one of those things really