I confess to a distinct sense of unease, as I think I recognise the stone TC and Jane put back up.

In 2003, Mrs H and I did the same thing to what looks like the very same stone*. Obviously we didn't do a very good job. I remember at the time there was wonderment on our behalf that such small stones had managed to stay in place for so long, and concern that they could be so easily knocked over. I suspect a stumbling sheep would suffice to put the stone back on the horizontal.

In retrospect, I think that at this particular place, the best one could do in face of a fallen stone that you deem in need of being stood up, is to photograph like mad, and pass details on to RCAHMW (or whoever). As has been pointed out, such small stones, once toppled, are quite unremarkable, which surely makes them more likely to be removed/moved further. I'm pretty sure I recall a few small stones piled in a heap, but only the one had an obvious socket, so was the only one that was evidently part of the circle.

Hob

*This is the circle in the valley of the powerlines, just to the left of the path as you're heading up from Rowen? Stone nearest the wall?