You would need to contact the insurer that categorised the vehicle and advise them that they have made a clerical error. (they will advise you that they will not speak or deal with you). You need to advise in writing that you will be pursuing a claim under the "interference of goods act" for the "diminished value of the car as a result of having a Cat C marker against it". This may cause them to change their mind, look into it for you and remove it. However, the vehicle was not yours at the point it was categorised, the statute of limitations which allows you to "seek rise to a claim" is six years which has now passed. This means you have no legal entitlement to do the above (but the insurer may not have cottoned on to this when you contact them) so worth a try.