A rebuild at Palo Alto Speedometer cost $195 around a year ago. Let us know how much in the UK. Smiths magnetic speedos and tachs seem to wear out the bush where the drive cable attaches then the needle does the bouncing thing.
A good story about this is the Triumph rider that was pulled over for speeding. The officer asked "do you know how fast you were going"? The rider replied wagging his finger back and forth...yea, about this fast.
I had pretty good luck with joellevinecompany.com. He is in Atlanta, Georgia. He gave me a quick turn around and exchanged green globe gauges for the propoer MK III guages I needed. Helpful to talk to and easy to deal with.