I live near the thames (not in town). There are several streams etc near me as well as the river itself, so for me I have an abundace of unclean water.
From what I have seen the Katadyn Pocket Microfilter Water Filter seems to be a good idea:
http://www.katadyn.com/en/katadyn-produc...yn-pocket/
Would that be good enough to filter water from a main river such as the thames, bearing in mind it has been found to have that Weils disease
http://www.bbc.co.uk/news/health-11625889 ?
If i'm gonna spend around £250 its gotta do what is says on the tin.