Managed a quick visit here following a call to the thoroughly decent farmer to ask permission to wander the fields. This later involved a bit chat about the placement and possible meanings of the carvings. It was this chap who first discovered the carving in the early 1970s, he says he's scoured the area for others, but none have turned up.
The carvings itself is placed in such a spot that there has to be a high likelihood that it's creator was making reference to the natural feature of The Piper's Chair. As Beckensall says, it is very finely carved, it put me more in the mid of the Galloway carvings I've seen than of the general style in Northumberland. Quite tightly made, and unusual in this neck of the woods to see 4 penannulars and such a shallow cup.
It's also puzzling to try and work out what the damage was. I'd always assumed that it must have been quarried, but on close inspection, this seems unlikely. It's really just surface damage, but deliberate or accidental, who can say?