Gah!
http://www.themodernantiquarian.com/post/65235/images/long_meg_and_her_daughters.html
This is very annoying. Not to mention illegal. Hopefully this is a one-off. I know it's a previously carved stone, but ffs. You wanna make a cup-mark? Go get a blank bit of stone! Make an unspecial stone special. Instead of damaging an already very special stone.
A pox on all Stone vandalising feth-heads.