You might find it easier to pour the base first, then the pier. If you pour the two within (about) 3 days, the concrete will fuse to a homologous unit as it hydrates. You could put in a "kicker" to give a starter for the pier, which will help locate the pier later.
It will make it easier to get a good finish on the base and to get all the concrete well compacted.
The pier will have quite a pressure at the bottom, so you need plenty of support, or the concrete will simply blow the shuttering out. Personally, I'd be tempted to make some frames out of 3 or 4 by 2 to hold your shuttering together. I don't think I'd trust the OSB rings to hold.
Finally, I'd put some support in for the pier shuttering and wouldn't trust that it will float on the concrete and still remain vertical.
My thinking on that was to use wire loops to hold the shuttering together - but I will take you advice I'll add some additional containment support and a vertical supports. At the base I'll use wood struts to hold the pier up in a set position these will extend out beyond the hole and then the vertical supports will then attach to them.
The base will be attached to the shuttering.
My plan is:
1. Dig the hole
2. Place broken bricks at the bottom, test place the components and mark the top level of where the base concrete should be.
3. Place rebar in (this will have been bent & tied into shape already.). It should sit correctly.
4. Pour base - using a patter to move it as it's being poured to help release air bubbles.
5. Once at the desired level, stop and give a final patting and levelling out, have lunch & cuppa, then give a second patting/levelling
6. Place former with the support structs etc. in place, add weights to prevent the concrete from blowing out.
7. Pour pier using a stick to move the concrete around and to pat it down inside the pier former. Use a torch to look down there lol.
8. When at the top - insert the threaded rods bolted into position with the top cover - this provides the basic level and spacing for the bottom brake discs (the discs are used as a levelling system with adjustable bolts between them to provide a level mount point.)
9. Leave for the remainder of the day
10. Place tarp plastic sheet over the top to protect from any heavy rainfall on the pier and base.
I'll be around to keep an eye on it - I have a few days vacation I need to take. After a couple of days I think it will be safe to leave to stand on it's own to finalise the setting.
Any idea how long I should leave the former on? Longer the better? My thinking is that after a 4 days it should be ok to strip the former off. So if I pour on the tuesday.. I can strip it off on the following sunday.