I've used a tarp by these buys before... http://www.ddhammocks.com/products/all_tarps , it was lighter and more expensive than a cheap blue one from Halfords. I've slept under it a few times, only once in torrential rain and it seemed fine.
Only have experience of the Tab Siltarp 2, and it's light, well-made and keeps the rain off. Much better after I added shock cord loops to the webbing loops to take in any overnight sag.