It is probably the Cypress aphid which is decimating Leylandii hedges. If you've got large areas of browning this is likely to be the culprit. Although any insecticide designed for aphids would kill them off given that this is a hedge we are talking about it really isn't feasible. Added to that the brown areas wont regenerate, you may just have to remove the hedge and replace it with something different... Personally I would love our neighbour's hedge to be infested with the aphid even though it is kept at around 2 metres in height. Cutting it back each year makes a huge mess and damages the plants and shrubs growing near it. From the photo it looks as if your is a golden leylandii which the aphids prefer... Sorry I can't give you any positive advice!

Anyone know where they sell Cypress aphids?

And these are ladybird larvae that are feeding on the aphids. The Cypress aphid is tiny!

Oops yes should have said that! Thank you Fractal, do you know of any real solution to the problem?

Very difficult to treat them effectively. You have to spray into the interior of the plant where they hide through the day. They move out to the edge (foliage) at night. I suppose you could spray at night?

It's damaging a lot of Leyland cypress, particularly the gold Castlewellan form which seems to be more prone.