Subsequent to my last message and replies for which I'm grateful, I
contacted the nursery in Cornwall. I'm sure that they will not mind me
passing on their advice.

"It sounds as if your plant is suffering from Phytophthora, a fungal disease
that is soil and water-bourne. (a relative of potato blight) This causes
die-back on parts of the plant as you describe. Many plants, shrubs and
trees are affected by Phytophthora. Unfortunately there is no remedy except
to dig up the plant, cut out affected areas, prune back and plant in another
position. This should do the trick. If you are a member of the RHS you
could send a portion of the affected plant for analysis. I think this is
free for members. A plant on one of our disply beds showed exactly the same
symptoms - we dug it up and potted it and two years on it is fine and back
out on display."
"Hope this helps"
"Sue Martin"
"Duchy of Cornwall Nursery"

Regards

Geoff