folk may well have done the same as me, and typed up a reply that was so totally un-useful that it never got posted.

All I can offer is thoughts/questions that will have undoubtedly occurred to yourself: i.e. They don't look old, but what the hell are they doing embedded in a wall like that? I can see the freestanding one as maybe a modern megalith, but why ones in the wall.

Somebody will know. Somewhere. Personally I suspect farmers put stones up like that just to confuse people like us.