Our last car was a focus and we had the same problem ...apparently this is a common fault which proved to be rather expensive and involved a new speedo. Seem to remember it feature on the internet.
I think the problem was that you had to buy a whole new part rather than just a small bit! Anyway it cost us £200 about 5 years ago.
Sorry.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.