You could fix a put a wooden floor to it and screw this down into the concrete base

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If you are going to put a concrete foundation in, simply cast short pieces of timber ACROSS the concrete surface, say 2 along each side and screw down to this when hardened. If not perfectly level you can easily put packings where needed under the shed base.
If not permanently fixed, form a square with 4 lengths of say 4x2 (100 x 50 mm) or better, fastened securely together and put the shed on here.
A damp proof course is a good idea in both cases.

Many Thanks

Another possibilty is to flatten the area and put some 2x2 flags down - either just to stand the shed on or the whole floor as mine is.
Depending on the size of your shed, you could use concrete fence bases as my friend has done. These are 6ft x 1 ft and maybe 4 of these would suffice. (6 ft x 8 ft)

Thank you for comments:

In the meantime I looked at those timber bases on line (for those who have lots of money) and noticed that they were using metal pegs/spikes roughly 440x22x22 with a hole for screws at the top (obtainable from Wickes) so I have opted for these, only £2.02 each