I may be totally wrong, but common sense tells me that I don't think you have a case. The fact that you can't have it 'legally fitted' isn't the seller's fault. You should've made sure of this before you ordered it. It was sent in good faith and they MAY be willing to have it returned, less a certain amount for their office work, but taking them to court may not be the best way of doing it.
the model I was sold was as recommended by the shop as being suitable for my requirements. One of my requirements being that I could use the thing..............