Update, Update, Update.
Something Overthehill said triggered my memory about a something I read about YZ 85 slides and the fix was replace the whole carb with one from a kx 85. Apparently the slide and needle are way wrong in the standard Yamaha carb.
So onto ebay I went, secured a carb and fitted straight up, and to our complete jubilation (mine and the young fella's) IT WORKS.
He can't beleive the difference in throttle response and best of all we've been riding every day on the same old plug. before the swap we would have gone through 3 or 4 plugs by now. Fingers crossed it's all clear sailing from now on.
Thanks to all for advise.