There is always a problem with leylandii.They are overshadowing of either your own or a neighbours garden, they suffer from windburn from which they do not recover and they also suffer from growing very fast which means you need to keep on top of the growth or you will be left with an unsightly mess which will be expensive in either your time or cost wise to have someone else remove them. As far as I can see the only thing going for them is that they are relatively cheaper than the alternatives at planting time. There are many better alternatives. There is a link to a site which may explain your browning being so widespread throughout the area. http://www.dailymail.co.uk/news/article-472999/Plagues-aphids-turn-leylandii-brown-Britains-gardens-climate-change-starts-bite.html