This photograph is of the hot and cold-water pipes running under the footpath on the west side of Tooting Bec Common.
The pipe on the right of the picture is for cold water. You can see that the workmen have put a blue rag on top of it and marked an object blue in the foreground. The black pipe on the left must therefore be the hot-water pipe. I would imagine that part of the upgrading work may be to lag it.
Cold-water taps are always on the right, and hot on left. The logical conclusion is that the taps must be to the south of this hole. Indeed, Tooting Bec Lido lies about 1km to the SSE. The absence of lagging on the hot-water pipe may explain its consistently low temperature, even in the summer.