Reinforced Concrete - Covering a Hole

Fair point. I guess the 6x2 would be within the cistern, not actually spanning it, so that it can just be knocked away with a lump hammer after the concrete has set. And you would leave the OSB behind, so that ultimately the cap might settle by 18mm once the trapped bit had rotted away?

I think I have seen a well capped by the corrugated iron method, where there is no way to remove formwork.

It's always good to explore different options, even if some of them turn out to be daft!

Perfect excuse to buy a mini digger, though! (Although perhaps too small for your job). When I moved into my present house (which needed quite a lot of work) I did think about getting a second hand cherry picker (surprisingly cheap for something "out of ticket"), although in the end that wasn't appropriate.