Aye Smudge,
Been in the loft dealing with a leaking ball cock!

As for your fencing mate and knowing the 'past life'

then the following should help.
Materials; rather than cheap SS thinnings, dipped in a carcinogenic green substance, carted across the Irish puddle for you to rot the *rse out of in 5 years:
http://websites.uk-plc.net/Plasti_Limited/products/Recycled_plastic_fence_posts.htmYou are correct in thinking 1 third below and 2 thirds above.
If you use the recycled product, then I would go with the concrete as a dry mix at 1 part cement: 2 parts sand: 4 parts ballast or purchase the ready mixed/rapid setting stuff at a bag a hole!
If you go for wood, use treated 100mm dia round posts rather than square sawn 100mm x 100mm posts, the sawing slices across the fibres and weakens the post structure. Place a 'pad' stone in the bottom of the hole, set the post on top, then pack loose stones/broken slabs, concrete around the post. If you can put a flat face stone on the winward side and then pack a large wedge stone down the leaward side with the bar it should all tighten up, keep using the 'button' end of the bar to ram the stuff in. Once it is all tight, put a good dose of creosote around the bottom and let it run down through the compacted rubble.
Alternative solution maybe to go for the redundant telegraph transmission pole solution and insert using the above method. Bit big, but it'll take a whore a lot of puff. Also consider a more 'open' or lower fence, rather than the dreaded 1.8m (6") 'larch lap' which act like a sail. Decking boards are cheaper than fencing panels (+ a go old lick of creosote) and I would check local sawmills/fencing suppliers for discounts. The market is competitive at present!

I would definitely use the old GPO shuvhole and a good rabbiting spade. If the auger wanders offline or greets a stane your bu88ered. DONT USE A POWERED AUGER, IT'LL TEAR YOUR CORE MUSCLES APART AND FLING YOU SIDEWAYS LIKE A RAG IF IT 'CATCHES' A STONE! BAD BACKS

Long may your clay puddling continue, I've spent many happy hours digging holes in the stuff, trying to encourage silviculturists to do the same!

Hope all else is well and growing.
Regards, Bill.