In reply to LJC:
I have drunk some pretty horrid looking water from the beacons and up in scotland. My advice is clean it best you can I strained it through my t-shirt boiled it and add some purification tablets (ebay is probably the best place some of the military surplus ones) shake well and you'r off. I was always told that the harmful bacteria are invisible and not to worry too much about cloudy/dirty water if you have taken the above steps. Obviously alway try your best to find running water and you should be fine......
Hope it helps?