don't bother, just contact the DVLA, tell them you have new clocks and make them aware of the new mileage. Clocking cars (changing the milage) is illegal.
Changing the numbers on a set of clocks is illegal
changing the clocks for other clocks is not illegal, but DVLA need to know cos when you have your next MOT the mileage won't add up.
Ok sorry, I had just heard that it was checked, hence very few people stick a drill on their gearbox to make the car lighter on the milage when it comes to selling