You will need to check the root system. If there are no nice new white roots then that could explain the struggle to survive. I had two, newly bought fruit trees, this year that did not have any new root growth and did not survive. Alternatively it could be a survival option in that it sheds leaves to reduce moisture loss until it gets established. Or it could be a virus brought in by greenfly or the like. Or as you have said, again trying to reduce moisture loss, drying winds (although hard to believe this year) and wind rock. Does it look as though there are any viable new buds where the leaves have dropped off? If so you may be all right. You should, I think, check the roots first.
Good luck.

Thank you for your prompt attention to my problem.
I`m a bit reluctant to check the roots as Its in a deepish trough next to the other healthy plum.
However in the light of there being no new shoots where the leaves have dropped unless another suggestion is forthcoming I will have to dig it out.
Thanks again

If you have two plums in the same trough it might be wise to pot them up individually before they get any bigger and the trough they are in now will not do for long.
Hope your second one does recover.

Thanks you for your input Steragram. I never thought of that. I was supposing that if they were dwarf the roots would not be so demanding and the trough would be good for at least 3-4 years. So it may be I will put them in a permanent position in the garden and see if the week one improves.
Thanks again