Will it work?

the main concern comes to my mind is watering/drainage. 

Can't remember where I read it but I think it does not do the soil any good. Guess the humidity might encourage the growth of all sorts of mold / fungus.

Thanks everyone.. I decided against the poly sheeting it and go for two layers of weed supressing membrane.
I looked under a sheet that has been down for a few weeks just out of cusiosity and there was a family of slugs underneath it.
I would have thought with all the hot dry weather we've been having lately it would be far too hot for anything to survive underneath it?
Oh well.... live and learn....