If it is aphids, you may be able to find some trace of them. My neighbour has the same problem and was told be a tree surgeon that it was a disease.

I was not so sure, leylandii need plenty of moisture and we tend to forget this. If you have them in a place where water is not getting to the roots this could be the cause. My neighbour has paved areas close to his hedge and I am sure that this was the cause.

Severe trimming can also cause the problem.

All the leylandii hedges up here in far north look exactly as you describe. There is no doubt that two bad winters have done this. So, I would say the more likely culprit is the weather.

Thanks for the replies

Its probably caused by the conifer aphid, check for signs of sooty mould aprox three inches in from the tips, the conifer aphid has been very prevalent in the last ten years, on my rounds over the years i have always identified this browning as being caused by this aphid, once dieback occurs then it will not grow back, prevention is best achieved by spraying the hedge with provado bugclear aprox 3/4 times a year.