Gotta say Slumpy that ,IMHO the Walderslade Wood stones aren't man made rock art . Not all cup marks , like stone circles , are circular but these have very sharply defined skewed edges totally unlike cups . Very uncommon to find RA on vertical surfaces too. I don't know sarsen very well but would guess that these types of markings are not uncommon and seeing as sarsen is a type of sandstone are possibly solution holes .