Thanks for the replies!
Pruning the more vigourous varieties to even out the
canopy is definitely a necessity from what I've read but there are a few accounts of established grafts dying-out despite this. It's possible that it may just have been a weaker graft or bad positioning that left that variety at a distinct advantage or a slight and undetectable injury may have set it down a bad path - who knows.
I'll definitely be looking for a tree that has equally-sized grafts for each variety but I wonder if the position of the graft has much to do with their long-term success? I noticed that some grafts are made directly onto the trunk of the rootstock while others are made onto branches. I suspect that the ideal tree would have all of the grafts made one way or the other.
I think it's a bit too late in the year to be moving grafts around but if I find one with good structure I'll probably try to move the trunk grafts out onto rootstock branches so they all get an equal flow of nutrients.
Phil, the tree of 40 fruits is really spectacular! I've been looking for technical articles on the varieties and the grafting rationales but I think the grafter/artist just keeps sticking them on where ever they seem to make sense. It could be that a graft on a graft on a graft is just as viable as a graft on a rootstock. A 4-in-1 with two additional grafts on each plus two more on each of those grafts would yield 28 varieties on a single tree!