It's a big clue that it's a square, because there aren't very many squares in South London. Having said that, if you call that square a square, it's yet another example of London's non-Euclidean geometry. :)
I have found a square to which all the clues make sense. Where I'm looking but I can't bloody find the building!
If you picked the right square, there's a clue in the picture which should make it trivial to find the building.
@Tenners started
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It's a big clue that it's a square, because there aren't very many squares in South London. Having said that, if you call that square a square, it's yet another example of London's non-Euclidean geometry. :)