Thanks to you both.

I had considered making a shallow foundation, filled with sand, over a layer of tough, builders' grade plastic, with some cement spread on the sand to then allow ground moisture to set the cement. But it all seemed a bit chancey! Especially as the layer of plastic would, presumably, prevent the incursion of the ground moisture! I think my main concern is getting everything level.

I'm wary of using slabs as flooring as I've been warned about, "dry heat," being a distinct problem.

Much food for thought!