I thought I better have a look at my sproket retaining nut. Took the cover off and my god it was filthy behind it so gave it all a good clean up and regreased all the joints etc. I've still got the old nut by the looks of it, it's solid enough and looks fine but will replace it anyway.
So went to my local yamaha dealer and got the "I've not heard of this before" so gave him the part number and the service code (from page 2 of this thread) and he said he'd give yamaha a phone to see if I can get it for free, he's ordered it anyway though and it's only £5.43 or something so can't really complain. Will be going back in a day or two to pick it up and hopefully not have to pay for it! haha