I just had my nut & sprocket replaced by Yamaha free of charge - thanks, Yam!
This is on an '00 Fazer, medium to high mileage, and scruffy in appearance. The only cost to me was the fuel, and 40p for a missing cover bolt.
As for the nut, if in doubt, phone Yamaha UK up. If there's no record of any warranty work being done on yours, chances are they didn't do it. If so, you can go to a dealer and have it inspected and replaced for free. Insist that it's a manufacturing problem Yamaha know about, and are replacing free of charge!!