The corner posts of the build were set into concrete before starting this project. But there are a couple of uprights that had to be added after the base was laid.
We used some heavy duty M10 anchor bolts to strap some 100mm x 100mm gate posts onto the concrete. These hold the post(s) pretty securely. But even with a load of strapping, holding a two-and-a-half metre length of 100mm x 100mm post perfectly vertically can put even the heaviest duty metal bracket under strain.
While it feels pretty solid, if I were to lean on the post, I could feel it shifting slightly. The wood remains straight, but the metal of the bracket, anchoring the post to the floor, can flex slightly. So, wherever possible, even when securely braced with metal brackets, I much prefer the security of a big grey slab to hold everything in place!
We used postcrete to hold the posts in place which goes off in about half an hour. Normally with postcrete - according to the usage instructions on the bag - you dig a hole then fill it a third full of water (the post and concrete mix go into the hole, with the powder added until all the water has been absorbed). This method has worked just fine for our fence posts in the past.
But because the shuttering (few bits of rotten fence panelling, crudely nailed together) wasn't water tight, we couldn't use this approach. So we added a layer of dry powder, then poured water over it. Then more dry mix on top and a splash more water, and then waited a little while to allow all the water to "sink through". Repeat until the shuttering is filled to a suitable level.
Although the instructions say the post should be fixed in 20-30 minutes, we left it overnight to ensure it was fully cured (working in temperatures of just 1-7 degrees means we're giving all our concrete much longer than usual, to go off fully).
The end result was a good, solid, rock-steady, perfectly vertical post. Although it's possible to push and pull the post about (with quite a bit of force) the base of the post is not moving away from vertical, it's just the wood, flexing along its length.
With all the uprights secured, it's time to get on with building the carcass!