Robin wrote:
...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.

you'd have problems getting a 12 inch gravel board in after they've been
concreted in -(the post concrete should come up to the underside of the
gravel boards, so that any future ground movement / washing away of soil
etc, cannot cause the panel to slip down on one side or the other) and even
then you'd only gain an extra 6 inches underground....what I've done in the
past with this type of fencing is to knock in old slates to a depth of
around 16 - 20 inches, although this was to deter rats, which has succeeded


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!

White ones yes, brown ones tend to warp given that they absorb sunlight and
get too hot...unless you can find solid white pvc posts...