A concrete base is always recomended for sheds but for a small shed, if you dont want to go to all the bother of concrete foundations, I think it would be okay to prepare a firm level base with hardcore then put a layer of builders sand down and then lay paving slabs on that. Some 4"x4" wooden bearers across the width with strips of roofing felt where it contacts with the shed base will be worth-while to stop any rising damp. Good Luck Tbird+