Holes in the leaves of my cherry blossom tree

It sounds like bacterial canker. Full description and treatment options here.

Oh no - I've noticed holes in the leaves of my young cherry tree too.  Been out to have a look at the trunk but can't see any oozing or dead patches.  Could there be any other reasons for holes in the leaves?

It's the most common cause, but it's one of the earliest symptoms, so if you give your tree a couple of sprays as described in the linked article, you should be able to stop it progressing.