My speedo unit seems to work (electrics) all except the most important thing- measuring how fast your going.
I've just bought a new cable and taken of the gear at the wheel. I spun the wheel and the cable moves so it must be the actual speedo.

Are these repairable or is there a common fault which tends to go on them? no doubt to replace it would cost a fortune.

Any ideas?

most common bits to go are nylon bush and thin washer under it, found at wheel end of cable. but if the cable is turning it could be the speedo, might not be so expensive as XR400, and 250 and probably more hondas have same one, so a breakers might be possible. i would double check the drive at the wheel 1st though, in case its working until your on the bike going faster.