Dave,
It's actually quite easy to alter the mileage.You'll have to open up the clocks u got and take out the speedo(middle clock).Now it's just a matter of turning the numbers until u get the desired on at the front i.e.where u'll be reading it while driving.Then u just put back in the clock and put everything back togather again.

Some people think this damages the clocks and the numbers don't line up correctly aftrewards but I've done it successfully on 2 RS's(after engine changes to reflect the actual mileage).

Best of luck
Jimmy