There are three Ordnance Survey Benchmarks on the sarsens of Stonehenge - two are on the road side of the Heel Stone on the righthand side as seen through the fence - one is 70cm from the ground and the other 20cm - both are quite easily seen if the light is right.
The other is 80cm up on the south side of Stone 16 - the pointy one on the opposite side of the ring to the Heel Stone - It is only visible with difficulty and not from the public path.
This is the Stone 16 Benchmark being pointed to:
The details of the benchmarks are:
OS Benchmark Search:
SU 1230 4224 CUT MARK (LIVERPOOL DATUM) NE FACE HEEL STO SW SIDE RD 101.3460m above sea level(at Liverpool - the old system)
SU 1223 4218 CUT MARK S FACE STO NO16 STONEHENGE 103.1138m above sea level
SU 1230 4224 CUT MARK NE FACE HEEL STO SW SIDE RD 100.6998m above sea level
POST UPDATED AND EXPANDED http://www.sarsen.org/2012/09/stonehenge-benchmarks.html