Hi- I am also looking around SE1, preferably Waterloo or Southwark for a room (full time share though). I have tried Gumtree - but that is a zoo. It is competitive and people often to call you back when you respond to their ad's or the rooms are taken.
If anyone else knows of any rooms, bedsits, flatshares available please please email me. I am a professional female (easygoing and tidy), and am willing to pay 100-200 pounds depending on the location and the flat.