This delightful little enclave is protected by the London Squares Preservation Act 1931, this space remained a private garden for #17 until the late 1980s when it was purchased by the Durand Gardens Residents Association. It was once laid out as a formal garden, but it is now maintained as a woodland garden.
This London Gardens Trust article has wonderful detail about the history of the garden and this Oval History article adds to this detail and presents historical maps of the area as well as interesting comments from past and present residents. This Brixton Buzz article gives us a peek inside this resident-only garden.