St Georges Gardens
Today's photo is another contender for London's most hidden park, You can see it in this satellite photo, but you won't from any road. I've known about it for years (I used to live near here), although it was TWWSI that sent me down there now.
It's an astonishing place, the first burial ground outside of a church dating back to 1713. It changes from a crowded picnic green at lunchtimes to a secluded nature reserve. If you are in the area, it really is worth a visit. It's that well hidden, it looks to me as if even the diamond geezer hasn't got here.