Yes or no?
If yes what to use?
Will be used (depending on the results of this highly scientific survey) in conjunction with goretrx army bivvy, tarp and mat but I intend to put the mat in the bag. Thinking in wet the bag would wet out, get everything else wet and be a general pain in the proverbial.
Thanks for your help/ and or confusing or contradictory advice gang!

Tyvek or Polycro are generally the cheapest options. Tyvek's harder wearing, Polycro's lighter ... both work.

I tried that polycro stuff but found it fiddly to put down flat especially if it is windy as it flaps about. You could peg it down I suppose. I'm using a groundsheet I bought from eBay that allegedly has thermal properties too. It's worked well so far. You can trim it down to save weight.

http://m.ebay.co.uk/itm/281593005878?_mwBanner=1

I'm never understood the need and havent had any issues in never using one.

It seems like the inverse of taking 1 thing to do two tasks