Web Master:
Hi Ian
This is a really difficult one. How is the drainage in the area where the hedge is suffering? Are there signs of insect infestation on that one plant? Is is possible that someone has tipping a chemical in that area of the hedge?
When you took the dead plant out, what did the root look like, were the roots growing outwards or in circles? Did the roots look healthy? Were there signs of insect activity on the roots, any Curl grubs? It could be that that particular plant was unwell when it was planted and its death has nothing to do with the conditions in your garden. Before you replace the plant I would wait a few weeks and observe how the other plants are faring, just in case the plant transferred a disease to its neighbours