Any clear plastic will work, but white, black, or "foggy" plastic will cause problems for most veggies, which need maximum sun to bear properly, especially in the UK! Horticultural plastic is usually just of a thickness to last a bit longer, or sometimes treated to resist damage from ultraviolet light, for the same purpose. The very best brands will let some ultraviolet light in, since that is beneficial to plants, but they can be hard to find.

thanx very muchfor the help i won't be buying any then as the plastic i got was completely clear. I'll give it another try. i bet your problem isn't sunlight but water?!

True, though sun can be a problem on the wrong side--my neighbor there has so many shade trees they're blacking out a quarter of my lot! Shouldn't complain, though, my cooling bills are much lower because of it. : )