I need to create a Hardstanding for the zed, to give me a good proper space for maintenance, gravel sucks.

As a rough guess 3mx2m would be sufficient to cover a zed foots print? I read I need around 4" depth of concrete or 120mm, that equates to roughly 1m3 of concrete.

I used a calculator and it's telling me i need

250kg concrete

400kg sharp sand

800kg of aggregate

That's a lot of materials for such a small area is it not?

Not that bit no. It used to be lawn but I needed another space so I cut the lawn out and used a load of shingle that was in the back garden I wanted gone. Just as a temp measure. So under the gravel is just dirt.