Walking back along the track from the burnt mound I looked to my left and saw what looks alot to me like a saddle quern.
I have seen examples of saddle querns that are much smaller. But have also read that some are quite large and set in the earth. I wondered if this might be one of those.
The “rim” across the bottom is more obvious in real life.
Considering that the area is absolutely peppered with hut circles it seems that it could possibly be a quern.