What I didn't know is that if you click "Request History" you get details of every electronic MOT the car has had. It tells you where it was done, details of any fail or advisory points and the mileage.
I looked up my Xantia and discovered it shed 70k miles between its previous MOT and the one I bought it with
This is a free service and after my experience I would advise anybody to run a check before buying a car. Apart from mileage you can sometimes tell if a car has been neglected by the fail items.
You need the registration number and either the MOT number or V5 reference number. Be suspicious of a seller who won't give you these numbers.