Have the leaves been chewed, or is there webbing between the leaves or have the leaves remained on but died? It's a bit difficult to comment without a close-up.

Originally it just looked a bit blighted and dead but looking at it again today it is covered in green and black caterpillars, a lot of the stems are stripped and it also has webbing. I've added some more pictures to the original one I put up

That's better, so pick off all the catapillars and webbing that you can and then spray with something like Ultimate Bug Killer. If that does the trick then give it a feed with Q4 or similar fertiliser.

Thanks so much. I'll try.
Any need to treat the soil beneath?

they are the caterpillars of the box tree moth.
Pick off as many as you can find and if you have a bird table offer them up on there. Birds will be very grateful at this time of year.

nothing needs doing to the soil as the eggs are laid on the underside of the leaves.

read the RHS link
https://www.rhs.org.uk/biodiversity/box-tree-caterpillar

if you can find the time to fill in their survey request it would be helpful.

Also, next spring, you might want to plant a pot of sweet alyssum and Viola tricolor nearby. Those flowers attract gnat-sized stingless wasps, which lay their eggs in caterpillars, eating them from within. Whether or not they attract the right kind of wasp for box tree moth is chancy, but the other species will probably benefit the rest of the garden.

Thanks for all your advice everyone. I'll try these remedies and see if they works. I see a number of sites also recommend growing different plants to box if it can't be controlled.