Had a good dig around the bas and root, removed bark and no sign what so ever of any Honey fungus.
Also no signs on the hedge or around of any aphid concentration or problem.
I tend to think it is a combination of old age, lack of feeding (there had been an old hedge on top this same bank before the leylandi were planted in the 70s) and the harsh winter.
What worries me is the spread from the more exposed end, but that could all fit with the above.
My suggestion to the client is to remove the dead trees, feed the remaining ones and sit back and see what happens. We can then take a really good look over the removed trees and have any testing done.
Thanks for all the comments, I will update when we take action, just waiting for client approval to get on.