It's a marvelous place in lovely neighborhood. just a short walk from Belsize village or Oriel Place and Flask walk. Buses ar near too, metro at 8-12 minutes walk, but noteworthy the area is quite hillly so good for your condition.
For those Sherlock Holmes's among us who notice there's a railway nearby... there is not, it's underground and you won't notice it. the apartment is very quiet, aside from sometimes the sound of people walking on the wooden floors upstairs and some normal sounds of people living. But we're really keen on our rest and during our stay it was really quiet.
The apartment is on the ground floor, but provides access to a backyard via stairs, which is lovely and very practical, extra nice when traveling with kids aged 2-11: the trampoline.
Sleeping info: the beds are good. In the room with the bunk bed the matras would classify as a tad hard, the single boxspring as medium, the boxspring in the master bedroom, I would describe as medium too. The one duvet should not be fought over, it's comfy for two persons so share it with love.