Our stay in Hector's South Kensington flat was wonderful. The listing is exactly as expected. We stayed in late September and the temperatures were pretty moderate, inside we were perfectly fine. I did crack the window in the bedroom each night for a bit, but it was a little noisy to leave it open (the noise drastically reduces with it closed). The beds are firm, but this is typical of this type of accommodation. The sheets are very comfy. You'll need to bring your own toiletries and may need to pop to the Tesco for a top up on supplies if staying for more than a couple of days - but it's only around the corner.
I'd highly recommend staying in Hector's flat and will do so again if traveling with friends.