If you have not done so already we would start by contacting the auction house with your complaint. A category X car should have sustained little or no damage while a category S car has suffered structural damage, so there should be a significant difference between the two. If this is unsuccessful you can take the auction company to small claims court if the value of the vehicle is less than £10,000. If you have the original sales information that shows the vehicle was listed as category X this will be a crucial piece of evidence in your favour.