Rev counter started misbehaving ie,idling at 0 but still working one day,back to normal the next day and now not working at all! (the rest of the cluster works 100% as it should) 

Any ideas folks? 

Could be something inside the cluster that's come lose etc but you would need to take it apart to find out and it will be quite fiddly inside.

Easiest solution is probably to replace but then you have to think about mileage correction etc.