The problem is that, at the moment, the top of the concrete is only one course below the DPC of the house, so I'm extremely wary of raising the floor level by much. Admittedly the shed should be reasonably watertight - the roof is flashbanded to the brickwork, and the base has been sealed to the concrete, but I still don't want to risk doing anything which might risk rising damp.
The other problem with your suggestion is that, at some point, the shed is going to play home to my motor bike - and when it's balanced on its centre stand, that puts a load of around 250kg through two small (maybe inch square) steel pads. I don't think flooring chipboard would last long under that!
Regards,
Martin