Builders merchants are miles better for anything, I often wonder how places like Wickes stay in business. They're bloody expensive and the quality is hit and miss at best.
Pics, sounds interesting.
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.
It might be worth looking at hiring a vibrating poker to get the air out... it's a lot less effort than tamping it by hand. I'd be concerned that a mix that is wet enough to work easily will be too fluid to properly support the form for the pier and weigts and as you are planning.
Usually, you'd expect to ease the shuttering a day or two after pouring and then remove it completely a couple of days later.
You're right to cover it protect it from rain until it's set. In summer you'd also want to stop it from getting too dry for the week or so after the initial cure, but I doubt that will be an issue this time of year.