Hedge Disease?

It seems like I'm only ever asking for help on here!! 

We have a corner plot and along with it a pretty huge boundary hedge on all sides. We love the privacy although it takes an age to cut! It's very established and stands 8-10 feet high as it wraps around the front.

Recently we noticed a couple of areas of the hedge that are starting to yellow and have some brown spots on the leaves.

I'm concerned, not only about that area, but the risk of it spreading. The thought of having to think about replacing it has silly cost implications I'm sure!

I'd be grateful if anyone can identify what might be going on and if they have any advice to treat it.

Thanks in advance.