maybe!!Could it be part of a sluice or drainage channel, being near a ford, the water may have been diverted in the old days, as you have said you have found other pieces, could be an early drain into the close waterway. Be interesting to find out what was on the site in past times.
Call me obtuse, but is a Cup shelf for literally putting cups on?
We haven't seen this piece of stone yet, TeeDee
Is there any obvious mortar staining on it? Has it got a crisply cut or a crumbly, ragged edge?
There are Devonian Slates that were used extensively for buildings; I'm not sure of the quality though - I believe they were coloured (not grey)
There's other bits??!!!
Have I got this right; you are in Wales? I lived in the Rhondda as a kid ... there was slate all over the place. But I think mainly from the north and mid Wales - I don't remember seeing slate quarries in the south ... blue pennant quarries everywhere there was house though
O.S. maps go back to the mid 1700's. Bound to have something showing over that time.
I wonder if is it the stuff that fell my great great grandad's cart when we where fleeing from the anti Beard pogram in the late 1850's ( There was no wick-ped-ia in those days so don't bother to look it up.) As such you are duty bound to pack it up and send to our house. ... cos my nan says it's the law init.