The theory. A Sheffield stand needs half a metre or more of space either side of it for the bicycle wheels, as in this picture below.
The reality. Because there are no national or even London-wide standards for siting Sheffield stands, highway engineers overlook the need to provide space for the wheels and love shoving them right next to street furniture. And then to make matters worse, the Council's own employees dump grey LBWF street sweepings sacks next to them. Hoe Street today (close to the junction with High Street).