Visited 20th April: I visited this site in the hope of finding something of the burial chamber that once stood here. Lou dozed in the car while I ran around like a loony with my GPS. I found nothing but sheep in the field where the tomb supposedly stood. There was a slight bump in the field, but nothing that I could conclusively identify. Perhaps for someone with a bit more time and patience there might be clues to find here, but in the end I gave up.
My post-visit research at the NMRW came up with the following about this site's demise in 1844:
The Rev. T.G. Mortimore remonstrated with the destroyer [of the stone], and with the view of inducing him to desist reminded him of the old saying that ill-luck befell those who destroyed the Druid's altars. Some years afterwards the vandal admitted that the house he had built of the stones had not brought him good luck!
At least we got some good folklore out of this site.
Some superficial research at the NMRW clarified the history of this site, and made me feel a lot better about not finding it. According to the Royal Commission's records...
This stone, known as Coitan Arthur, Arthur's Quoit - probably the capstone or supporter of a demolished cromlech - stood in the corner of a field appurtenant to the farm of Trellwyn Ucha. It was destroyed in the year 1844.
I'd like to thank Phil for adding every site with the word 'Arthur' in it that he could find to the Modern Antiquarian Web site, including this one which hasn't existed for 159 years!