OK folks. Here are the missing cup marks on Robin Hood's Stone (or at least, that's what they appear to be):
http://www.themodernantiquarian.com/post/27716
I didn't spot them while I was on site, but when I saw the 1927 photo posted by Paulus I realised they should still be visible.
I've contacted English Heritage and they're going to look into. If I'm correct, and these are the cup marks, then EH will have to amend the NMR for this site, because it states that the cup marks are "buried in the soil beneath the concrete plinth".
K x