The chair is together but it's ended up rather higher than it was :P a bit too high if I'm honest, since my feet cannot touch the ground when I'm on it.
So I have to figure out how to lower it. The shaft on the hydraulic strut which holds it up has a shoulder which rests on a thrust bearing, then the bottom is held in place by (essentially) a circlip.
I may be able to drill out the base a little, find a slightly larger thrust bearing and weld it in place further up the shaft, or thread the shaft and use a nut to support it and one to hold it in place at the end, then cut off the excess.
It's very comfy and stable, so I am pleased with the result, once I lower it a bit it will be perfect! It's SUPER heavy - I picked an office chair with a heavy cast base deliberately so it wouldn't be tippy.
Some shots/videos (reverse chronological order):