From the Dyfed Archaeology record -- the site (which I initially thought to be unrecorded) is in fact mentioned by Murphy and Wilson 2012, and is given the reference number PRN 100702.
Here is the description:
500 metres (that's a mistake -- it should be 50m) to the east of PRN 100700 lies a small sub-circular enclosure measuring approximately 7.0m in diameter (PRN 100702). It is of unusual construction, being made of very large stone blocks, some of which are set within an earthen bank. Incorporated within the eastern side of the circle is a large stone slab lying in a horizontal position, that has been levelled by balancing it on several smaller stones placed underneath it (see photo below). The stone slab is 2.6m long and 2.0m wide at its widest point. This site had never been recorded before, not even during the field survey in 1984 undertaken by Pete Drewett that recorded sites in close vicinity to it such as PRN 100700.
The area of Banc Llwydlos has been little studied in the past but this fieldwork in 2011 and the field visits undertaken in 2009/10, also as part of the scheduling enhancement programme, have shown the high quality and rarity of prehistoric sites in this area. 500m to the east of the two sites described above lies an already scheduled hut circle group (PRN 1565, SAM PE370) and 300m to the southeast lies another hut circle and enclosure group recorded in 2009 (PRN 14373) that is of equal importance.