It looks like Box blight.
See the link
https://www.rhs.org.uk/advice/profile?PID=1005

Looks like box blight - a recent fungal infection of box and difficult to manage. See https://www.rhs.org.uk/advice/profile?pid=96 for fuller info.

Oops Hywel - your reply wasn't there when I replied. At least we said exactly the sam thing...

We must have posted them at the same time :D