sorry but you got sold a car with a busted speedo unit.
it will cost you £500+ to get a new one and have a main dealership re-program it to the chassis number of your car.
i'd suggest you go back and raise merry hell about it and get a partial discount on the repair costs.
no you cannot nick one from another car, they are all one shot coded.
Thanks for your reply pick the car up Friday, the speedo works fine in miles but the reading for how many miles the car has done is in klm i just thought you could tell the computer to display miles. dave