Damp problem with shed. see below!?

I have had a wooden shed built and it is on top of a concrete base. the problem is when it rains water runs down the outside of the shed and underneath it between the wood and the concrete. thus it is always damp and the floor has mould and fungus growing on it. is there any magic diy stuff i can buy from say b&q to fill in the gap between the wood and concrete and remove/treat the mould?
Answer
Ok, you know how not to do it. when i put my shed up(on a concrete base) i lined up bricks to take the bearers, then removed every other brick (creating gaps in the lines) then, over the bricks i used a heavy polythene damp course, then the shed on top. you know, i dont have a damp problem.best thing you can do is to disbloketle the shed, and start again, properly this time..