I spoke to someone from Keepers Nursery, it seems pretty conclusive from what he said that it was bacterial canker. I hadn't even noticed all the little shot holes in the leaves. I knocked a small branch off myself while weeding earlier in the year which is probably where the canker got into the main trunk. The guy at KN told me had the tree been older it would have been able to survive it. It's made me very cautious about replanting with another Jubilee since he said it is susceptible to canker. Such a shame. I've got an Amelanchier lamarkii that I'm going to put there. I'm not happy about where it is at the moment so it seemed an obvious replacement. I won't get the plums but the blossom shouldn't be too different and I'll get fantastic autumn colour.
If you do replace your plum with a plum keep in mind replant disease, you will need to dig out a good hole and replace the soil from a part of the garden that hasn't had plums grown in it and use micorhiza. Of course if you replace with something other than a plum you won't need to worry about diggin out any soil. I have six other plum trees in the garden including a victoria so I won't miss the fruit.
Talking about supports. I really don't bother, we were told at college back in the ??!!'s that for most trees a stake is a disadvantage. The Horticulture community is very conservative though and most people still sugest staking. If the tree's on Pixy then you may need a stake but make it as low down as you can and use those rubber spacers to keep the tree away from the stake and put the tie as high up the stake as you can. Keep in mind the stake is only to steady the roots not support the tree so the stake should go deep in the ground.
Thanks for the reply.