GOOD NEWS! Before taking rod (welding) in hand, and risking spatter on my o-rings, I decided to give those nice people at Yamaha a ring. Turns out a mod should have been carried out during a service at some point and, assuming affected parts were out of tolerance, this - and any resulting repair work - could be done now foc.
I got the bike into a local dealers (Skellerns - Cheltenham - seem very efficient) who took measurements of the parts, sent them off to Yamaha, next day Yamaha came back saying yes they will foot the bill.
So, if anyone is still suffering with this problem, get your local dealer to check the bikes history. It's much better than risking spatter!