Whats my milage ?'

If your car is new and has had no prior owner, you can simply calculate the figure off the clock, remembering that it is "kilometres travelled per year" that you want (divide the amount on the clock by the time - in years - you have had the car).

Otherwise, you need your 2 latest MOT certificates. Subtract the figure for "total km driven" on the older certificate from that in your most recent AND THEN DIVIDE BY 2 - the remainder is the figure you want.