Andy,

I've seen "cup" marks like that, and of the same size in Great Langdale in Cumbria that aren't considered to be prehistoric.
The reason is that it is an area of volcanic rock. Just today I saw similar marks that were only the diameter of a 2p piece. There were dozens on a rock the size of two footballs, and similar ones all around in the mountains that surround the valley. Many more exist that are the size of the ones on your photo. Sometimes it's hard to differentiate between prehistoric and volcanic cup marks. The volcanic ones are obviously bubbles of trapped gasses.

Difficult to decide what is what in an area of volcanic rock, but it may be simpler elsewhere.

Cheers,
TE.