I haven't seen the book myself so I was just assuming that he had visited the stones.
If they are just mentioned in the gazetteer then yes, he could have got the info from somewhere else but where from? I can’t imagine him just adding them to the list on somebody else’s say so unless he was sure that the source was reliable – he has a reputation to uphold after all ;-)
After I posted the fieldnote I had a quick check for information on the nearby Roman camp - it’s mentioned in several places but the sites I looked at made no mention of our mystery stones, so I’m guessing they’re probably not Roman but as Kammer suggests, middle ages/monastic or something similar. If we could get hold of the scheduling info that should sort it out.

-Chris