Hello - welcome Mark
. Firstly, can I ask why the water is not draining away. What is your soil like? It sounds as if it might be heavy clay. Even after heavy continuous rain here on chalk, the water drains within a day.
Assuming it is heavy clay, I would dig a succession of holes around the shed in the wet area a few feet deep and fill those with grit to start with. Just aeration with a spike or fork will help, but in a bad case larger plugs would be beneficial. After that's in place, I would just put a minimal amount of grit on the top to make it look posh!
It would be a good area to plant some climbing roses and honeysuckle. Having plants there would be very beneficial to aid the drainage.