File:London Stone, Yantlet 338.jpg
The corresponding marker above London is London Stone, Staines at 31 km as the crow flies upstream from London Bridge. But this stone at Yantlet is 54 km downstream.
Direct access to the stone is difficult because Grain Marsh is an MoD firing range. To take this photo, I approached from the British Pilot in Allhallows and paddled through Yantlet Creek. Here are my feet on the way back. More images.