Tom thacker wrote:
I have a mature yellow plum tree which in recent years has been
developing a problem causing the fruit to malform soon after forming.
The problem has inreased over the last ten years. When the fruit is
near ripening late July 50 percent of the remaining non deformed
plums rot on the tree resulting in a loss of most of the crop. The
trouble has only started in recent years I never had the problem
originally. I haven't been able to obtain any help from various
garden centres, any answers.

Tom

This sounds like a fungal or bacterial attack on the fruit. You need to
find out which one before deciding on treatment. Any chance of a picture?

D