ss wrote:

I know there are all sorts of preservatives in B&Q etc for painting on
fence posts but isnt there a longer term solution like coating the
part below ground with something to stop water soaking in to the
posts, like a plastic coating. I am basing my enquiry on the basis of
thinking most of the bog standard preservatives are so enviromentally
friendly they probably are not really much good.

Any thoughts or ideas on this.

Use galvanised steel posts instead.