When I picked my 340 1.7 GLE Ltd Edition from Gainsborough train station in 2003, after buying it unseen off eBay for the princely sum of £250, the mileometer was working fine. However, a couple of miles down the road, having filled the tank, I had the bright idea of re-setting the trip counter - when I did this it stopped the mileometer working and neither have budged since! I have probably only done 500 miles or so though, she's been SORNED for all but 6 months of the last 5 years...
Any clues as to what has failed and what the remedy could be? The speedo is working fine... as it's only done a genuine 56K (or maybe 57 at a push) I don't really want to change the head if I can help it

Do a search, there was a thread posted a while ago about what goes wrong and how to fix them.