Graham,
What sort of ground is it underneath your deck, I take it that it's dirt/rubble? You would need quite a long post and a deep hole, seeing as the top of the pier will be at least two metres above ground level (I think you mean 1200mm).
I'm currently building a pier through the back deck. Mine is in to sandstone and not far below the deck, so my total pier height from the bottom of the hole to the top plate is around two metres.
The digging of the hole is shown in this thread:
http://www.iceinspace.com.au/forum/s...ad.php?t=58252
The metalwork has now also been completed, so I'm just waiting to glue it in, which will happen as soon as the epoxy arrives!
Cheers,
Jason.