Thanks for the reply mattylondon. I'm afraid as a complete layman I didn't really understand it!
So the mileage in a replacement cluster can be corrected to match the old one, that's good news.
This is the cluster that I saw on eBay:
http://www.ebay.co.uk/itm/1119945436...%3AMEBIDX%3AIT
Can I just plug this in and it'll work??