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
(This is an updated post with extra photographs added)