If the car has been given a Cat D marker then it will have been previously damaged and repaired by an insurer. It is not illegal to repair and resell Cat D cars but the dealer must disclose any known damage to the vehicle when showing it to any potential customer.

I would advise taking the car back to the dealer that sold it to you and demanding an explanation. If you are happy to keep the vehicle then you can negotiate a partial refund to reflect the status of the vehicle and lower market value. Or you can demand a full refund, minus a fair deduction for the usage you've already had from the car.

If the dealer refuses to play ball then you'll need to seek legal advice. We would also report the dealer to Trading Standards.