What sort of plastic do you use? Does it matter?
Yes. It has to be thick enough to stop light getting through otherwise the weeds will skulk on under it, you are probably looking at something around 1000 gauge. Black is the preffered colour as this blocks light better. Being fairly thick also helps because, left out in the weather for a few months, thinnner types of plastic will often rip due to the wind. Ideally weigh down with rocks in the middle and cover the edges with soil. Ideally you want something a bit more sturdy than a load of bin bags gaffer taped to gether
I've used green tarps from Wicks (£5 for 5x3m) with cardboard underneath - this worked very well as the cardboard kept the light out and the tarp kept the card board dry.
You'll get some people that say that you shouldn't use plastic as it stops water getting to the soil. I've never had a problem with this as it still seems to soak in from the sides.
You can also get the weed repressing fabrics - these tend to be a tight mesh of strands so that they let water through but keep light out. But be warned, couch grass, in my experience, will grow straight through it given time.
Bodger, I am new here, don't suppose you go on the AG forum do you?
Fraid not, not sure what the AG forum is.