I booked Mark's place for my relatives visiting London and they were happy with the choice.
Even though the rooms are all very small (kind of standard in London anyway) the apartment is quite nice and clean.
The roof terrace is also a great feature, but just be careful with the stairs as Mark also mentioned!
The best thing overall is probably the position, even though is not very close to any tube stops (well 10 minutes by foot max) there are a lot of buses going in all different directions so you won't really even need to get the tube as much.
Communication was also great and quick, so if you need somewhere to stay in London for 3-4 people I'd definitively suggest Mark's place.