The Big problem at WK is that it is a reconstruction.
o one know which way arounbd the stones were meant to be, apart from a few obvious one's like the biggies that fell forward, as there are no sockets because they weren't buried like other Avebury stones.
One of the strange rituals I do now and then (Yes I do rituals and yes they are strange) is to walk around the stones in the dark feeling the surface of every stone, this shows up pollished areas really well, if only I could remember which bloody stone I found smooth bits on I could photograph them during the day.
I think the idea of the cup marks being ground on fyfield is very likely. I don't think the stone I took you to is one that has been 'modified' as the whole surface is smooth and not characteristic of other stones nearby with natural holes that could be ground down to form cup marks.
The stone I found on the IoW was sandstone and I could easily scratch into it with a fingernail. It was so soft compared to Sarcen (correct spelling) that it was almost like soft stone (that would be clay then eh?)

PeteG
Ps. Let me know when you are coming back here as I have a Sarcen trail of interesting stones for you to inspect for me