hollys not looking too good advice needed

[bradshaw groundcare]

Hello I maintain a large garden with a large holly hedge which over the last few years is only growing at the top. the bottom 5/6 foot has died off leaving the top 3/4 foot which looks terrible.

 

I also prune 5 or 6 individual specimens as shown in the picture a couple which look like the same variety as the hedge are also going the same way.

 

any advice would be much appreciated

 

I would say the hedge is best part of 50 years old