I have had this problem on both speedo and tacho and had them sorted by Speedy Cables who now operate out of Swansea. If you can't find an instrument repairer I can give you their number. I think it cost about £40 a time, but it is much cheaper than a new instrument. You can take them apart yourself, by gently levering off the black bezel which is crimped onto the body, but it is virtually impossible to do it without causing some minor damage or, as in my case, cracking the glass .
Having had them fixed a couple of years ago, there have been no more problems.
Terry
Trying to get out of the garage