Privet I'm guessing?

Progressive die back could be honey fungus. This opportunistic organism has two basic phases. Saprophytic where it does it's proper job of breaking down dead wood returning it to the soil as nutrients however, it's a bit of a rogue and will happily attack living plant material too gradually spreading through the soil in this progressive way.

Have a dig around the roots of the dead part and see if you can find anything that look a bit like black bootlaces in the soil. Sounds strange I know but the aggressive phase produces these structures (called rhizomorphs) to move through the soil towards it's next target.

One other strange feature of these rhizomorphs is that they give off a faint greenish glow at night!

Read more about these fascinating fungi here including there strange luminescence:

http://www.forestpathology.org/dis_arm.html

Hi Fractal, I am very sorry i forgot to thank you for your help on this question.
I did pass on your feed back to my Dad
thanks again :0)

No problem :-)