If no pest or disease is visible at the top could it be a root problem

Could well be vine weevil. Check the root system. They may have all been eaten . If it is weevil you'll see 1cm long white grubs in the compost

Thank you for your help, I might have to tackle the root system before it completely dies.

You could try taking cuttings of the green shoots at the top, just in case...

What a good idea, will do.