Gorgeous tree by the way! That's a healthy nice looking tree. Nothing to worry about. Many trees react this way usually right before a storm or with excessive humidity. It should eventually right itself. You don't need to do anything.

mine does this from time to time towards autumn. I assume it is a result of the leaf starting to prepare for leaf fall. they also tend to be where there are large clusters of berries and could be due to tension/torsion due to the weight of berries. the tree is usually fine the following year until about now.

Many thanks to you both! This is the first year that I have noticed it but otherwise the leaves are in great condition.