It sounds as though something (probably a small caterpillar or sawfly caterpillar) is eating them, and older leaves will fall at this time of year especially in very hot weather, but what is important is the new growth. Is it still looking good and being produced. Trees are used to being munched and will ride it out.

The same thing happens every year to our cherry trees, and it tends to looks worse than it is. We still get a massive crop of fruit, so I think the answer is probably "Don't worry too much."

Thank for the advice, it's much appreciated .