after a lot of research we used creosoted 'octoposts'. Little more cost but if they do what they say on the tin will prove to be long lasting and value for money (which from the research i have done appears they will?). Only been in 18months so can't comment further - other than that one was broken during winter storms by a flying roof sheet - and inside was good as new. Same company also I believe provide telegraph poles - agree with other comments about these. Proper creosote is the only worthwhile treatment - others have had the useful ingredients removed! As an option I believe people used to soak posts in waste engine oil - maybe some untreated posts and just soak the bottom bit to make the licking safe apart from tongue splinters?
We also used about 90 chestnut posts untreated which a local chap swears by - time will tell. But has to be better than HC4 (etc) softwood. We have some very old chestnut paling in remarkably good condition - but obviously above ground. last thought (but expensive - i looked!) recycled plastic posts are available.