Exactly. I don't have any particular trouble with the concept of large stones being moved and deposited, but since it is the Salisbury PLAIN, why should it be expected that the glaciers made a delivery of a pile of like stones in a convenient spot? :')
My neck of the woods are littered with glacial erratics. Some are in the middle of broad, alluvial rich river valleys, some sit atop hills and mountains. It would make sense to use a glacial erratic that was already sitting atop a small rise on a plain. It would stick out like a sore thumb for nomadic hunters as a landmark, gathering place. Wouldn’t take long for some to get a belly fully of elk and a little bit of fermented juice and then next thing you know Thog says, “Hey, hold my leather flask. Watch this.” And soon it’s a drunken project.