Might be Winter Moth damage - the caterpillars eat the leaves from bud burst through till late spring, and the holes start out small and enlarge as the leaf grows. Close inspection of the tree might reveal the caterpillars, or webbing between leaves. The adult moths emerge between autumn and mid winter - if it is that, apply sticky grease bands to the tree trunk by mid autumn to prevent the females crawling up to lay their eggs on the branches for the next year.

Or possibly even snails - amazing how high up a tree they will climb.

Thanks Bamboo and Steragram. I obviously didn't read the label on the sticky bands because i only put them on last week that explains why they have had no effect whatsoever. I never thought of snails either..Although there is a lot of damage i think the plums are coming on ok , i had some off there last year just a handful but they were lovely..