Our family of four had a wonderful week-long stay at Peter's flat in South Kensington. The location was amazing -- it was a 5 minutes walk from Hyde Park and 10 minutes to the Natural History Museum and the V&A. It was also only 5 minutes to the Gloucester Road tube stop. You could also get to a a small grocery, a coffee shop and various other stores without crossing the street. The flat was clean and had the right amount of space for our family.
I will reiterate a comment from a previously reviewer that there is a pub across the street, but want to mention that the noise was inconsistent. The first weekend we were there, the pub was closed before we went to sleep, but later in the week (which was during World Cup), people would spill out into the street for cigarette breaks until well past 11 pm. We brought a white-noise machine, which pretty much drowned out the outside background, but it would have been annoying otherwise.
The host was very responsive to all of our questions