Traditional bases of hardcore & concrete invariably cost more than the shed itself, and do seem to be overkill for a lightweight shed.
The reason is that any base is only as good as the ground that will be bearing the load. These bases are fine. Structurally strong, and rotproof.
It's simply a matter of taking a chance, Millie. To be sure though, I would certainly remove the turf, and also ram the subsoil well to consolidate it before installing.
I appears to be only £50 more for the concrete so I think I will go for that. I am worried that the plastic stuff will sink over time, I would rather pay a little extra for what I know.
Take a tip Sillie, Do not bother with plastic, the trouble I've had with our's is beyond a joke, Condensation causing a lot of the tools I have to go rusty, get a wood one.
Thanks for all your help, I gave had the concrete put down today, the shed has a wood base but I am planing to put Lino down also as I plan to work in there, does that sound OK?
Place Tantalised treated timber bearers on the concrete before placing the shed on top. This will prevent rainwater soaking into the shed floor bearers and eventually rotting them.