[Seajay]

Have you only just noticed it or was it there last year? A very bad infestation of red spider or conifer aphid could cause the damage but I doubt if either are around in any great numbers at the moment. Another possibility could be one of the cankers. They can run some distance up the side of the trunk and any branches contained in the area will be affected. You will need to have a close look. Insects can be controlled and canker is unlikely to kill a leylandii tree.
It is a shame as Castlewellan Gold makes a beautiful hedge if looked after and contrary to popular opinion is excellent for wildlife. I have one kept to 6ft and clipped to a wedge shape. It is like a marquee for birds in winter giving warmth and protection as well as insects.

Colin