Hi Simon, welcome to the gang
How exactly is your speedo toasted? - whats the symptoms? - Does anything work at all?
I did try swapping a speedo once from a scrapyard vehicle as my speedo needle kept sticking. The scrap unit barely lit up but the mileage display and the rest did come up clear as a bell. When i test drove it the speedo unit refused to budge and the mileage didn't do anything either. From that i concluded that it was either shot simply because it was a scrap unit or that they simply cannot be swapped. The latter seems most likely.
There used to be a guy i swapped messages with on
http://www.renaultforums.co.uk who used to fix speedo units for a living. Unfortunately it's been moons since i've been on that site and i have no idea what his username was. All i can say is that he was very knowledgeable and could swap mileages, security codes, reprogram chips etc.
Just in case you have the sticky needle thing and thats your only glitch, this is typically caused by the clear plastic cover fitted over the speedo unit. They warp with heat (the heater pipe behind is made of cardboard, splits open with age and warms the unit). The cure here is to remove the black plastic surround (starting with the long piece above it with 3 screws) and remove the plastic cover. Then remove the speedo unit itself (you have to play twister due to the cables) and take off the front plastic. A tad unsightly but then just leave the speedo unit uncovered until you can find a straight plastic cover. That or you dismantle the speedo, remove the needles and drill the holes out a bit bigger... then you need to make sure you somehow recalibrate it lol.
I've taken you off moderation so you can post away..
Saffles