How well have they been watered since they were planted and how deep is that slate?

The slate isn't deep at all, it's only on the surface. They have been watered well. Whole areas are thriving and really green and then other areas are full of brown plants. The brown plants seems to be spreading. So areas of the garden are either all healthy plants, or all brown plants

You can see the large areas of healthy box in the background of some of the photos

And the soil in all the planting areas is the same?

I don't know? I assume my whole garden would have similar soil but I couldn't be sure?

Can only suggest you look at the info on the RHS site, it doesn't look like blight to me but I am only looking at your sideways photos not the actual plants.

https://www.rhs.org.uk/advice/profile?pid=96

Not Box blight. I also thought water - either too dry or too wet, prob too dry. Buxus likes loose damp fertile soil. I would lift the slate and check the roots of the affected plants. If ok then add a feed and fresh compost then cover with slate again.

Ok thanks. Will do

I'd say a water issue - the ones near the house look healthy, is it likely they get more shade there than the ones in the centre? If so, that would confirm a drought issue - shaded plants don't dry out nearly so much. so when you say you 'watered well', what, precisely, does that mean? A walk round with the hose and a gun on the end once a week? A gallon per plant twice a week from a can? A sprinkler running for half an hour on each section once a week or every couple of weeks?