Good find. So I gather it is near the remnant of the causeway?
Yep. That's the Northern Boat Pit in the background.
I gather you think that originally the AE carved a perfect hole in limestone bedrock to accommodate the "pillar" and then later someone who wanted some limestone took a bit from alongside and it happened to break in such a way that very thin slither (original pic) happened to be left?
Most likely. The block in the red circle is smaller than the hole whereas the basalt pillar fits neatly in. I haven't counted them, but there are about 18 pillar holes in the "Temple."
It is difficult with all that rubbish around to make out exactly what state the pavement was in but it sure was a mess at that time.