Possibly box blight. If it is there is no cure I'm afraid.

Did you just give this bush a haircut? It looks like it was freshly cut. This could be the 'undercoat' that wasn't getting enough sunlight until you trimmed it. Leave it be for a while and it should green up.

Agree with Bathgate - you'd have seen dying back parts on the foliage before you cut it back if it was box blight. Leave it alone and see what happens.

I have two of these, one on each side of my arbour and I find the one that gets more sun is less green and looks a bit brown in patches. They are the same age, are fed at the same time, clipped at the same time, have both been well watered, so I'm putting it down to sunstroke! I'm hoping the less healthy looking one will recover after the heatwave.

It wouldn't do any harm to give it a feed (maybe seaweed extract)
I used to have a very long box hedge at my last house and because it was such a dry hot spot, some plants did go like this every so often.I would fed them or, when they were small, whip them out and put them in a pot and feed it etc. but, this one looks too big and established for that so I'd give it a feed where it is and, if it's very hot and dry where you are, give it a good water and spray the foliage with water..

It certainly doesn't look like blight to me!

Thank you all for the replies, and yes I did give it a good clipping two weeks ago and have watered and fed it. I will continue to do so and see how it goes ?