I've posted some pics:

http://www.themodernantiquarian.com/site/6999

It's next to the larger pond on Tooting Bec Common. I parked in the lido car park and walked over some sports pitches to reach it. However, looking at the map, there appears to be a nearer car park in Dr Johnson Ave, with a footpath leading most of the way to the stone, which is just to the north of the pond. You can see the pond in my pics.

Baz