Gotta say I'm having a bit of a tough time accepting that the OP couldn't tell a nearly 10 y.o. car from one 2 or 3 years old.
I'm having a MUCH tougher time accepting that the dealer couldn't tell the difference. I hope I'm wrong, but I'd bet that the dealer will not be willing to make any sort of refund unless forced to do so, and will try to fob you off, then if you persist keep his head in the sand and ignore you for as long as possible, hoping you will lose heart and give up.
If I'm right... Let's face it, selling cars is a trade that attracts a lot of .... not overly scrupulous people, shall we say..., and the OP has had his leg lifted. There's no doubt the car is a 2006, but unless the V5 says different, the dealer will no doubt point to the DVLA information -"The DVLA said 2011 manufacture and 2011 registration" - as his defence, claiming an honest mistake, and I can't see Trading Standards being too keen to prosecute in those circumstances. There's no question that the dealer is liable, but I doubt he will be in any hurry to deal with the complaint, and it will all drag on and on and on...
Klaus, I'm inclined to suggest that the best way to proceed might firstly be to let the dealer know what you propose to put on Facebook, and have put in this forum. Keep it strictly and totally factual, to avoid any suggestion of attempted blackmail. If that doesn't produce a result and you have to go to law, the best route would probably be the small claims track of your local County Court for the difference in value between what you paid and what the vehicle would be worth if not misdescribed, and for that you would need an honest independent valuation of your vehicle's full retail selling price. The small claims limit is £10K. Take professional legal advice before you go to law.
Taking full County Court action would be expensive, with no guarantee of complete success, and I think a small claims action would give you the best chance of persuading a judge that the dealer really should have known that the car could not be a 2011 model, and (equally importantly) that you could not reasonably have been expected to know this.
If and when you get a judgement against the dealer, you then have to enforce it. That's another story...
Good luck, and I hope it works out for you.