you have what sounds like a breach of contract - if you want to pursue the matter i would suggest enlisting the help of a solicitor - might cost a bit but would be a lot less expensive than losing the original car... it will probably only take a letter from them to get the dealer playing ball again. Alternatively simply telling the dealer you are enlisting a solicitor might be enough on its own.
though this is a verbal contract any court would reaslise a contract had indeed been made (proof usually being tricky to find) becuase to the evidence in your favour e.g. you have a witness, you no longer have your original car (car A), you have a written receipt / order form, you have the V5 to a car you do not actually posses, no one would relinquish control of an expensive car without getting something in return... further more the courts in cases with car dealers generally go with the member of the public anyway (guess judges don't like car deals much either)
good luck