Moveflat.com was always my favorite.
400 per month is low, but do-able. My last house share (i live with my boyfriend now), was £420 per month, but then add bills on top of that.
It really does take a while to find a house share in London, as its competitive and often places are meeting with SEVERAL people over the course of several days, and then get to pick who they like the best.
I found it was most difficult when I first moved here, and most places didn't seem to want someone who was brand new to London.
It took me a month, and it was only because my dear mom has a friend from work, who's daughter (who i had never met) lived in London, and let me couch surf on her sofa for 3 weeks, that I had a place to sleep. Not that you NEED to find some awesome person to let you live with them free for 3 weeks, but just be prepared for it to take a while.