I'm wondering where you are in the UK - these plants are not fully hardy, so in the Home Counties, for instance, in a warm, sheltered spot, they will do fine. Elsewhere they may suffer, so its possible, depending where you are, that the very cool conditions, particularly at night, have affected the plants. I assume they were under cover when you bought them - did you harden them off before planting out?

Hi
They are approx 3.5 metres high and planted round a
2 metre high brick wall to provide further screening.
We live in Surrey and parts of them are ok and although
they are dropping some leaves it looks like the leaves are literally being cut and eaten off the lower stems.
We bought them from a wholesale nursery and they were already stored outside and looked really healthy.
We understand they need early pruning to train a good shape and also think they may have birds nesting in a couple of them, but would there be caterpillars or such like that could be feeding off them?

Slugs or snails? Try slug pellets, but also, do you have rabbits there? Were it caterpillars, you'd see them on the plants if you inspected them.

Thanks. I will have to climb up to try to inspect further i think!