Hiya Welly,
I have the Trekmates Pak Poncho - £20 - from Cotswold Outdoors. (you can view online
100% Polyester, taped seams, map pocket and packs into it's own stuff bag
Total weight 279g.
This one piece also covers the rucksack.
The only negative I found in an 8hr deluge was it caused excess...