There was an item on Gardeners Question time about a disease that affects these trees. Unfortunately it's incurable and will kill the trees over a couple of years. Sorry.

I was under the impression that no one problem can be attributed to the demise of the trees...we're talking about the golden leaf variety 'Frisia' I believe.
On a slightly different note I have noticed an ever increasing problem with mice striping the thin layer of bark from the branches which in turn also causes severe die back, and can kill very young trees.