As you can see it looks suspiciously like a milestone, albeit in a strange place. The main road is a good few hundred yards away from its position and although the Common has been traversed by riders since its inception, I would have been surprised if they were using it as the main route anywhere.
And then... I saw it. Right on the top. A metal rod had been placed into the body of the rock in the middle of a slight depression and there, just to the side, was the faint remnants of three chisled arrows pointing toward it.
Thanks to M.J. and his information regarding the identical sign I saw in Brentford last week I was able to identify this as another benchmark. It's surprising how, when your eyes are opened to something for the first time, you seem to see them everywhere!
If you haven't read M.J.'s comment I can tell you that these 'benchmarks' are sited and used by the OS as points of reference for all maps. In effect they are the standard points from which all other measurements are made. The three chisel marks were historically used to site the feet of the surveyors equipment but I assume they are more symbolic these days. In this particular instance it's the metal rod that 'marks the spot'!