I recently bought an ex-Jaguar UK XJ Portfolio 3.0d from the local Jaguar dealer. On the day I bought it I had to take it back as the aircon did not work, the front wheels weren't balanced, and rear interior trim panels were loose.

It has since been back again to rectify various rattles and a problem with the TV tuner. It still has various creaks and clunks. When he showed me the car, the seller pointed out that the road tax disc had been purchased at £440, even though the car's actual emissions put it in a lower tax bracket. I told him that low emissions and fuel consumption were amongst the reasons I wanted the car. He assured me that there was simply an emissions figure error on the V5, that Jaguar would supply a letter to the DVLA and that the emissions figure would be corrected on the V5 so that when I come to tax the car it will be at the correct, lower rate.

I've driven several thousand miles now in all types of road conditions and I can't better 35 mpg, which suggests to me that the emissions figure on the V5 may be correct. What are my rights if the seller now tells me that the car is indeed in the higher emissions and road tax band?