They do allow for the changing of a speedometer. The system isn't as anal as you 
As far as I know the only way to 'correct the mileage' is to remove the clock and force it round with a screwdriver 
I know it can be up on the database WRONG

I want it to be right

You can dismantle the odometer,, bit fiddly, so practice on the old one first, you might be able to go old skool, and just use a drill to go backwards. 
Thanks , I'll try the code reader

on the old clocks (i know i can change mileage on LCD digital clocks like astra) and if that fails I will follow your advice and take the old clocks apart as you suggest

then learning from that ,what parts snap off etc

then TRY and do it right with the new clocks

If it's on standard 225/55/16 tyres, then your computer will be spot on 
Thanks DG , I matched part number on clocks and being ex plod ,i assumed the code index would be OK
just wondered how long it takes to get average MPG data etc back (saving zero ,but not driven it yet) range is correct, idle MPG correct ,just no average consumption or trip distance
I didn't have my wallet in my pocket so the wallet thickness sensor in the driver seat may be confused
