Thanks to the advice here, I collected a Keter plastic shed in the back of the car on Wednesday, and had it up by early Friday. My lovely wife has now progressed beyond her comparison of it to a big portaloo to saying it looks quite good. The whole job was done single handed except for the window, which did need two of us. I prepared the ground as they suggested by digging it over, removing stones and tree roots and then raking level. During this process, under some trees, I was surprised by the huge quantity of worms that appeared and the lightness of the soil. The base went on, and seemed firm and flat, so the shed was thrown up. This involved a lot of tramping in and out, and one corner of the floor near the door hinge compressed the soil beneath and became springy. I suppose I should have compacted the soil a lot more. I don't think the worms will have helped. Jacking the corner up and poking soil underneath with an improvised tool seems to have got the floor stable and the door locks lined up, so we seem good to go. My thoughts have now turned to wind.
In the past with sheds, I've hammered big angle metal stakes into the ground and fixed the shed to them. The instructions for this shed refer to 1 1/2" to 3" screws through the floor. If this refers to diameter it sounds huge and doesn't match the holes, if it refers to length, it seems rather pathetic. I'd have expected at least a foot into soil.
So to the question... are there such things as soil anchoring screws? If so, what does one ask for and where?