Thats a lot of concrete! were you doing it yourself? I've never laid this much before, I am, shall we say, apprehensive, about it.

But my first issue is whether I need to build retaining walls or not. the ground slopes up to the left and towards the back. The far left corner is 58cm higher than the datum (front right corner), Frnt left is 28cm higher as is rear right. So if i dig down 275mm (so the top of the base is 25mm above datum) I have a 'wall' of earth 855mm in the left rear corner. I think that needs retaining. and similarly the left and right side need retaining to about 1/2 way along the base and grading from there. Got to find a solution that doesnt take up a lot of space else I either need to move the base over a bit, or make it smaller... and I've already compromised on size :(