weve just done 18 of them 400mm square 900mm deep. used a hired hole borer to break the worst down and then the double spades to clear the spoil. very easy, more than enough room to get it out
gravel wont bind when you compact it. youd be better leaving it there and giving it a covering of mot. The cobbles can be laid on a sharp sand bed, but i would rather bed them in a semi dry mortar mix
we always wrap the frame in the membrane rather than putting it on the floor. stops stuff dropping through and weeds such as bindweed etc, finding their way up through