Susan's is AMAZING. The flat is in Zone 3, accessible enough that by the end of my week's trip I was walking to the Tube in a span of 10 minutes not realizing I had gotten there! While it isn't exactly Central London, there's a bakery across from the station where you can get a great breakfast for less than a pound and by the time you've enjoyed that and the paper, you're in the heart of London.
Susan looks out well for her guests and attends to so many little details that make the flat incredibly homey—not to mention SO WELL WORTH the price.
I always felt safe and very at home. For travelers seeking a real London experience—ESPECIALLY students—I cannot recommend Susan's flat enough.