Strawberry Tree Dying...

This tree / shrub has been in the garden very happy and healthy for 13 years but a few weeks ago the middle started going brown and dying off. I've just come back from 2 weeks away and it's got a lot worse. The shrubs behind it seem unaffected so I've ruled out anything being sprayed on it from the neighbours.  This is a lovely structured shrub in my garden which I really don't want to lose. What should I do to save it anyone got any ideas what could be causing this and what should I do?

any help very much appreciated.