...also you're way off with your measurements - you don't need 9 ft posts
for a 5 foot fence, you need six foot six ones.

I am very ready to believe I've dropped a stitch. But with the slope taking
up around 6" that would seem to leave me with just 1 foot buried. Is that
really enough, even concreted into good loam?
I was reckoning 5' fence + 6" gravel board, + 2'6" buried = 8'; but favoured
the larger posts to give scope for 12" gravel boards in future - eg if the
neighbourhood foxes dig even deeper.

PVC is generally crap WRT fencing, it suffers in direct sunlight and
becomes brittle.

More so than uPVC windows? I'd settle for them lasting 25 years!