A friend of mine has just bought a nearly new VW Golf from Arnold Cla*k, advertised as having 10,000 miles. When she arrived to collect it, she had a look round only to find it had nearly 15,000 miles on the clock. At this point, all she had paid was £200 deposit. She told the salesman that she did not want the car as it had been advertised incorrectly and didn't want a car with that much extra mileage, and asked her £200 deposit back.
The salesman simply laughed and in not so many words told her to do one. She complained, to which she was 'tossed' the tax disk and told 'thats got about 50 quid on it, have that'. My friend walked straight out and got onto trading standards, who said she was totalled entitled to full refund due to the car not being the same spec as advertised, albeit mileage and not specification.
She return to the main dealer, informed them of what trading standards had said, and got full refund. I know these are slightly different circumstances, but she found Trading Standards very helpful, and it got her situation resolved.
Good luck with whatever you end up doing.