What a horrendous situation to be caught up in.
I would have though, that if MMC provided the info about the car you were buying, then they have a contractual liability to deliver that car, with those figures. It's not up to you to make good the difference, no matter how this may have arisen.
At the end of the day, it's just utter bollox! Your car will make NO difference to global warming.
It's time we all got serious about addressing the root cause of all this tax raising BS... don't get me ranting! I will stop now!