Concrete-In fence post supports

[damian]

Hi,
I am about to put a fence up at the bottom of my garden and am wondering if you could give me some advise.
My neighbour put up a similar fence last year using fence post support spikes but he told me the ground is very soft, and his fence has started to lean with high winds we have had.

I'm looking at using concrete-in fence post supports because I'm using 75mmx75mm wooden fence posts and do not want to concrete the posts themselves in because if they rot they are hard to repair/replace, so this seems the best solution for me.

I'm wondering if you could advise me on what dimensions the holes should be that I pour concrete in to.
Should they be 24" deep by say 12" wide or is that too much?
Also, are the post supoprts supposed to be visible or is it ok to bury them below a layer of soil?

Thank you very much for your help.

Regards,
Damian