We had a nice stay at Mark's place. The flat is very central, well located and very spacious.
The communication with Mark was very easy and he was arrangeant with our arrival time (he gave us the key right after our arrival, so we could immediately enjoy our stay in London).
We were however a little disappointed by the cleanliness of the flat (tacky floors and kitchen worktop). It would have been great to mention that the single bedroom had no window.