Shed base not flat

[LES EATON]

I have recently erected a shed in my garden. For the base I used an area of concrete that had previously (before I lived there) been used as a shed base. Unfortunately, the recent heavy rain has highlighted the fact that the base is not level as the water does not all run away and a large pool builds up underneath the flooring.
Before you all say "should have checked it out before building the shed", I'd be grateful for any ideas to fix the problem without having to take the shed down. :(