shed foundations

I would be concerned about the lack of a concrete footing on the patio. If the patio is only 4" thick, the weight of the shed will most likely crack the patio.

The patio is monoblock and so is not even 4" thick. What would you recommend?

I would probably dig down to the frost line and pour concrete piers. One at each corner and one at the middle of each wall, eight total. You can set the height of the piers to allow for the ventilation you want and set then level. However, that may be more than you want for the shed. You would need a sill that would handle a 5' span.

You're in the UK right?

Is it "not quite level" because the ground slopes for drainage or because the patio wasn't laid on a proper base and it's moved?

The normal way would have been to remove the slabs/blocks where the shed is going and then laying hardcore, formwork and a concrete slab. Gets you a nice solid rot proof floor as well as foundations. Another less professional method would have been to construct a platform out of pressure treated softwood or perhaps old railway sleepers.