Steve Rainbird wrote:

We have a Damson tree (Merryweather) in our garden which has a problem.

Some of the leaves have become very "scrunched up" like a screwed up piece
of paper and some of the fruits are turning yellow. Does anybody have an
idea of what it is and what can be done to help it.

Silver leaf disease. It's a fungus that affects the Prunus family.

http://www.agr.gc.ca/pfra/shelterbelt/shbpub44.htm