Malcolm wrote:
This year, it grew as vigorously as usual and was smothered in blossom
then, about three weeks ago, we noticed the leaves were turning brown
even though the flowers, initially, didn't seem to be affected. However,
they soon were and the whole plant has now gone brown and the leaves and
flowers are falling off. It has, to all intents and purposes, apparently
died, though cutting with secateurs reveals the centre of the stems are
still green.

When we moved into this house we had a bird-sown one on one fence,
rooted in a few inches of soil between the fence & hedge. Prolific, it
were, and no bother. I used to give it the bowl of potato-peeling
water most nights in the summer. After 15 years one dry, hot summer it
upped and died, much as you describe, and by autumn the stems were dry
and brown.