I bought an aftermarket clutch cable on e-bay made by HI-LEX, which claims to be made "under license".
It fitted perfectly and felt much better then the original which has started to fray near the lever. I took it for a test ride and was lucky I didn't go far. It quickly felt loose and the more I adjusted it, the looser it got. I was fortunate to get within a mile of home before it failed completely. There is a manufacturing defect which means there is no crimp in the liner stopping it from sliding up the metal elbow. The only thing there is a tiny spur of metal where the end of the liner has been bent out. It works fine until the cable warms up and experiences vibration, at which time that spur slips into the mouth of the elbow and liner follows it. The harder you pull the clutch, the more the liner slides up the elbow, shortening the sleeve and increasing the slack in the cable.
The pictures show what I mean.
I got lucky, you have been warned. Be careful what you buy.
In the name of research (and because I really don't want to pay £60 for the Yamaha part, I have ordered a replacement from Wemoto they claim is made by 'Slinky-Glide'. I will let you know how I get on.
