A brand new cluster is silly money from X part, Rimmers etc. SMC on Ebay can sell you one at a much better price.

http://cgi.ebay.co.uk/MG-ZT-V8-Instr...item2ea9701c87

I think the car will fail the MOT if the milometer is faulty so you need to establish if the 99999 is the actual mileage or a fault!

Do your homework on your key, proper ones each have three parts, blade, mobiliser chip and remote fob for your car to run you need blade and imobiliser chip, if the chip is missing then somebody has done something nasty to the cars security system and you may have extreme problems insuring it without the oem system working as they ask the question on the forms

Here's an idea, you have a standard CDT, do you have two keys and if so do you have the bar codes in the owners manual for those keys then one can be coded to your car, ideal as a temp measure