Go to
http://www.gumtree.com Flavius. It's probably the best website for finding flatshares, as it's free and squillions of people use it. If you're looking for cheaper (ie: £400) I'd look at South of the river. Put a search in for the area (stick a tube map in front of you and look at places which are central enough but not so central it'll cost a bomb to live there).
Good luck!