The stone with the flat face toward the camera appears to bear a simple single cupmark.
Interesting observations SG and Nigel.
Just had a closer look at the third stone from the camera (towards the top-centre of the stone). Don't think we should get too sidetracked on what may or may not be cut into them though - either way they're still (possibly) very, very important items that once formed part of the intrinsic fabric of the Silbury structure.