Hi

How old is the hedge ? and how large ?

It may be a aphid aka greenfly called (cypress aphid)

As you know its been so mild so that wont help , try and have a look closely at the foliage or put a white piece of paper under neath and give the branch a bash and see what falls off .

I think the only thing you can do is spray once you find out thats what causing the brown patches.

Depending on conditions, it may need water.

Certainly wouldn't be water shortage here in Wales!
Another cause could be wind burn if its in an exposed position. And there is also a disease around for which I believe there isn't a cure so far - hope it isn't that!

Conifers are very tough plants and can live for hundreds of years. Very few things bother them. However one thing they can't stand is too much water. Have you been getting copious amounts of rain like I've been hearing about? Check to see the water isn't pooling around the conifers. Are you conifers sitting in pools of water or muddy super saturated muck for extended periods of time? This is the bane of their existence. They are drowning. They need a chance to dry out with only the occasional rain fall. "Brown patches" indicate something below the surface. Excess water can precipitate a host of other problems - root rot, mold or fungus disease, all kinds of pathogens, bugs, etc.