So no, of course I don't mean my first post to the blog, but we did concrete the first house corner post in today. We weren't able to lift the post into the hole so used a kind of gin pole arrangement.
Then using a hand-crank boat trailer winch we pulled the cross forward, lifting the post upwards.
We bought some aggregate on the mainland at TOP$10 per sack. It's crushed coral rock, which is of course limestone, except quite a bit harder than what we're used to in NZ. It took a couple of dinghy trips to bring 3 sacks and a bag of cement over to the island. We bulked it up 50/50 with sand we collected from about 20 meters in from the high tide line so it wasn't salty, and chucked in a quite a few coral rocks to bulk it up a bit more. It took a good chunk of the day to collect rocks, sand, water from the well and get the post raised, braced and concreted but getting the first corner pole in made for a good day.