As stated by Stubob, cup marked stone and channel/groove, with concentric rings.
Like a quarter of an oval stone, the 'squared off corner' having an obvious area with 2 concentric arcs, ie. a quarter of another cup and ringed structure... clearly this stone is part of something much bigger.
The curved edges seems to be deliberately manmade such that I believe it to be part of an oval cist slab, with multiple cup and ring and simple cup marked areas of rock-art.
I've had the chance to examine the stone very thoroughly, measurements taken, recorded accurately, to photograph from all angles with oblique lighting, etc
Two decorated stones were found at Ramsor Farm in the late 1990's.
The first, a sandstone slab found while digging a drain, appears to have been broken off a larger stone and is marked with 5-7 cups.
The second found 30m away in the foundations of a wall consists of a cup mark and gutter surrounded by 4 rings and a group of two more similar rings. Badly worn this stone also looked to have been part of a larger design.