Hi QFF,
Regarding slab thickness, I'm guessing 10mm was a typo and you either meant 10cm or 100mm... I any case I would be tempted to lay it deeper than even 100mm.
I currently work in structural landscaping we would do a concrete pour at least 7 or 8 times a month or more, when we are paving around a swimming pool our concrete base will be at least 100mm thick or greater and that is for foot traffic only, no vehicles and no structures, just a layer of paving tiles on top.
I assume the job is costing you plenty and if it is worth doing its worth doing well (etc), I suggest seeking the advice of a builder or engineer regarding slab (& footings) depth as there are many factors involved, including but not limited to:
soil type;
soil density;
surface water;
ground water;
type & weight of building;
type & weight of traffic on the slab;
etc;
Anytime we do slabs we err on th side of caution.
Lets not forget that a slab of this size will need good solid reinforcing with a good steel reinforcing mesh such as SL92 or SL102 sat on chairs.
I advise taking all this into consideration and then taking the advice of Bendo and the Doc, get the schematics from your shed supplier and soil tests done on site.
Hope this helps.
Cheers,