GeorgeJungle

I should add : take out the spark plug and put some fuel in. Then it will fire and run. This will prove it.

Exactly the same problem here. Was in the (useless) Uno Motorcycles @ Shepherds Bush for 2 months , they thought it was a wiring loom problem and proceeded to change it! Apparently this is a well known problem. It is something to do with the fuel additives in UK fuel that degrades the internal seals in the fuel pump. The main symptom is not starting at all after being left outside, however mine did not do that it was simply having fuel starvation problems (flatspotting, running rough). Took it to a Yahama dealer (Slocombes in Neasden, there are few dealers about these days even in London!). Said £400 repair but Yamaha are making a contribution as the bike has <4000 miles on it. Bike is 4 years old. Fingers crossed they will pay at least half!