Nice flat, easy entrance. There was a mistake in the pin code sent to unlock the door, but they called me back less than 10min after I texted about it.
If you like to walk, you can access the center in around 30min. Otherwise, the underground is 5min away from the flat. Just far enough to get a take-away coffee in the street :)
Once in the room, we never heard the trains going through the Waterloo station (free ear-plugs were hidden in one of the drawers anyway)
Only flaw : no heater in the room (except a small one in the bathroom) - for a weekend in December we got a bit cold