make a written record of everything including the dates when you called/phoned the dealer.
you should go to a CITIZENS ADVICE BUREAU and they will negotiate on your behalf with the dealer. they will give you unbiased advice, and tell you straight if you have a case or not.
if you need to go through a small-claims court they will advise you.
or you could go to your local TRADING STANDARDS department.
they will all do this for free.
be warned - this kind of thing can take months to resolve.