We had a wonderful stay at Mark's apartment! The only minor hiccup we had was when I left my phone on the Portsmouth bound National Express, but we were easily able to connect with someone else's phone. Mark walked us through all of the details of the house as well and told us to call if we had problems (we didn't).
The apartment easily fit our group of three, and was able to fit my sister, who at one point stayed with us, as well as a couple we know who stayed on the inflatable air mattress. The bathroom was able to fit my wheelchair in too, albeit tightly.
There's only one key, so don't split up before you leave the apartment, although our group accidentally did this twice and Mark's wonderful neighbor was able to lock up behind us. The location of the apartment was great too, we could easily get to the Russel Square tube station (literally right outside), a Marks & Spencer n(right downstairs) and a 24-hour Tesco (around the corner).
I highly recommend staying here!