Whats killing the Privet??

Hello fellow gardeners, I need your expert help and advice.

One of my customers has a privet hedge which appears to be dying, very slowly, but definitely progressing.

I've dug around the roots and I'm fairly sure its not honey fungus...not showing any of the fungus signs.
I'm not sure that it's wilt either.
Under the bark there appears to be some sort of white fungus looking layer, but this might be rot setting in?

It's a fairly old hedge, so it might just be old age?

What do you guys think? Any help to identify what's happening would be awesome, and some advice on how to tackle it.

Many thanks in advance :D

 

Dont know.
Just wonder if it has been waterlogged at some stage in the past.

 

I don't think so, soil is fairly free draining and its on the top of a bank, it slopes down on the other side