Mileage mistakes are very common on MoT data. There is a window of a few weeks to get it corrected. I guess if you wanted to get heavy you could sue the MoT station for the 'damages' of putting the incorrect mileage on the MoT certificate that you could claim makes the car impossible to sell because it comes up on any HPI, Experian, etc MoT mileage data check. Assuming the value is less than the £10k small claims limit you could go so far as to demand that the MoT station buys your car from you at 'market' value for the make model and year at the correct mileage. No guarantee you would win.