Elena was super nice and let us check in early, and even had a travel cot (pack n' play crib) for our son, which saved us a lot of time trying to track one down. The location was spectacular, very close to great restaurants and the train and both Regent's Park and Hyde Park.
The layout of the apartment was great as well - the bedroom is bigger than it looks in the pictures, it gets lots of light and there's a very nice patio that you can sit on and have tea or a drink or something.
We also stayed at a 5-star hotel during our trip and Elena's place was MUCH better. Highly recommended.