I've posted a picture with dots, from memory, of where the grooves are. I didn't realise it had been so big and there might have been another circle against it if that bottom groove is going the other way around. The rock face is quite wobbly and uneven, it's not a millstone, and seems to be of a similar material to that carved stone at Knockroe (thanks) - a soft sandstone with large bits of white rectangular quartz embedded there. The style and technique of the grooves seems very similar, too, if not actually identical. I wonder whether any other bits will turn up now ?