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Yes the idea that it was a family tree was a reasonable guess, but you are right, it would have floered by now. It seems very ulikely that the orientation of tree has the effect you describe.
It seems to me that somehow, I don't understand how, the rootstock has managed to sprout above the level of the graft. I haven't heard of this before, but your tree has all those symptoms.
It's your call, but if it were me I would saw off the offending trunk. After all, a normal plum tree has only one main trunk.
Remember to only do this when the plum tree is in full growth, early June is about right. If you do it when the tree is dormant it may well suffer from silver leaf.
Thanks Dave I appreciate you taking your time to reply. As yoiu say it will be best to remove the useless bit as soon as safely possible At least then it may have more chance of success next year !
In some ways its a shame I cant use the offending trunk for a hedge its vegetation would be dense enough !
again thanks.. for help and a great website