Ok, thanks to serendipitously meeting up with Bladup, who was at the circle when I arrived, I can report that yes, the axe carvings do exist! Not carvings so much as bas relief, they're on the underside of the central stone, on the right hand side. Quite large, about the size of my hand.

I'd always looked on the 'top' side of the stone in the past, no wonder I'd never seen then :-)