The tax band is set when the car is registered: As yours was done after March 2006, you'll pay whatever the rate is for the emissons, so the £460 or whatever it is will not change regardless of what you do. Even if it was a 309 car, you'd still be paying that. You have no comeback there, so don't waste your time.
Against the dealer, it's a different story. Unless you have that it's a 309 car in writing (On the invoice? On an email enquiry?), I suspect that the law will not find in your favour if it went to court as on the balance of probabilities, it's likely the dealer did not tell you that it was a 309 car. I'm not calling you a liar, in fact I completely believe you, however I suspect a judge will not. As such, you're very much reliant on the dealership themselves to reimburse you maybe £1500 tops for your woes, but I'm guessing you'll be pissing up the wind here.
What I would do is pop down and see them, and hopefully the same sales guy is there. Ask him if he remembers you, get him to confirm that you asked for a 309 car in casual conversation, then get him to inspect yours along with one of the techs and then the ball should drop. At that point, you've kinda got him over a barrel. Do it all friendly like, as if you've just popped in for a chat, and give him enough rope to hang himself with. If you go in all b=guns blazing you'll get nowhere, as they'll just close ranks.