On a visit to Waun Leuci standing stone (5.6.2010), this is easy enough to fit in with no effort.
To find, if you’ve been to Waun Leuci stone, head back down to the road. Cross over the road (look both ways) and head straight across onto the grassy slope. Before you reach the bank of the Afon Tawe, the setting is visible right in front of you – prepare to be amazed. There are four very small “slabs” in a trapezoid shape, surrounding an equally small “boulder”. Joking aside, if this is what it claims to be, it’s unique outside of Exmoor and is presumably related to the ritual landcape (stop sniggering at the back) that includes the stone to the east, Maen Mawr and Cerrig Duon and various barrows nearby. I’m prepared to believe this is a deliberate placing of stones.
The stones here are quite small, but that isn't unusual for the Beacons. What is interesting about this arrangement is the shape and size. I'm not familiar with the West country Quincunx's that it has been linked with, but my rather incomplete info on them is that they are circle's/squares with a centre stone. This one isn't really either of those. It's more of a Cross shape, and a reasonably accurate one. The centre stone being approximately 6 feet from 3 of the stones, and about 3 and a half feet from the other one.
The only other Cross shaped stone arrangement I know of is Callanish. There are probably others, but still something of a rarity I believe.
A study of this little enigma brings up all sorts of interesting coincidences and possibilities, something that I missed when this first turned up. Definitely not an accidental arrangement.
Thanks for this Cerrig - I must admit it didn't exactly grab me, but it doesn't look natural. The difficulty is that there are any number of small stones scattered about here and it's easy to start seeing more things than are there - having said that, who's to say a natural arrangement of stones couldn't be viewed as significant to the people who understood and used this landscape in ways that we can only speculate on.
There are a growing number of reasons to believe that these stones are deliberately placed, too many to post here today. This links it to the other sites you have mentioned and more besides. When I have finished working it out as best I can ( if i ever do ) I will post it up somewhere.