There is no problem in pruning out the damaged /dead bits. Have a look at the cut branches is there any discolouration in the wood?

Thanks Seaburngirl. I hope to start tackling the job in the next few days and I'll let you know what I find. I suspect it has just come to the end of its time and that the non-dead section is simply the next generation. Maybe that's too simplistic! I will be happy if that's all it is, and I'm hoping no nasty surprises are lurking in the soil to affect the good part later on.

If it's any comfort mine does this, I just remove the dead bits and the rest grows as normal

Started cutting out the dead and dying bits. No discoloration anywhere, which I guess is encouraging. Having cleared much of it away I have been reminded, by seeing a couple of old stumps, that I have obviously done the same thing before, though so long ago I don't remember doing it! Most of the rest needs a better saw than the one I have, so I await my son's arrival to help. Interestingly, it's clear to see now, the dead part grew from a stem that divided into two a foot or so up from the ground, but everything else is on three relatively new separate stems with no divisions near the ground, plus a much newer one that goes straight up. None of that was visible until I started cutting.

Thanks again. Alan.

that is excellent news.