We enjoyed our stay at Katherine and Jon's place. They were very friendly and respectful and answered our queries promptly. The weather was quite bad at the time and my friend's flight was cancelled at short notice. We didn't know when he would be able to fly again and they allowed him to stay for free in case of further delays :) Other than being terribly helpful, they also offered us breakfast throughout our stay.
Location wise, the apartment is very close to Queensbury station and you can get to central London in just half an hour on either Jubilee or Metropolitan lines.
We would certainly recommend the apartment and the hosts if you ever find yourself in London :)