I asked an older colleague who was brought up in London what he thought the date of the lower photo was and he said 40's or 50's. But he also identified this photo as Devonshire House - Stratton House (the upper photo) is the next block along.
But this gave me an idea - I looked for images of Devonshire House and came up with this 1928 image where you can see Stratton House in the background.