So the first step was to locate some railway sleepers - in my view, far the best material for making gateposts. All that creosote should give them a good (second) lifetime.
These are in fact Spanish ones, and, according to the dealer who sells them, they are made of pine. As each one weighs about 60kg I suppose I should be grateful they weren't in fact oak as the now-unobtainable French ones normally were.
Once again the new trailer has been very useful. The sleepers are 2.6m long and although they overhang a bit I had no trouble getting them home from Rodez.
The next step is to dig some holes. I intend to find a man with a digger for this. The ground is far too stony for me to manage with a pickage, and I'm hoping to put about 75cm of sleeper into the ground.