http://www.britannia.com/history/docs/coggeshl.html
Margam Abbey Chronicle
The Discovery of King Arthur's Grave at Glastonbury
This entry from a chronicle of Margam Abbey is one of the accounts that have come down to us, detailing the discovery of King Arthur's body at Glastonbury Abbey. The date of composition is uncertain, but it is believed by some scholars to have been written within a decade or two of the original discovery in 1190. A much later date is likely, however, as we will see.
Margam is the only report that mentions the discovery of Mordred's tomb and is, on that account, suspect. The chronicle also mentions transferring the body of Arthur "with suitable honour and much pomp" to a marble tomb in the abbey church. The only reliable record of anything remotely like that happening is the account of the visit of Edward I to Glastonbury in 1278, when he transferred, with much pomp, the bones of Arthur and Guinevere to a marble tomb in the abbey church.
According to C.A. Ralegh Radford (the archaeologist whose excavations at Glastonbury have shown that the monks did, in fact, dig up a grave at about the right time), the relics of Arthur and Guinevere that had been exhumed, "lay in a treasure in the east range of the abbey" until the time when they were reinterred by Edward, some 88 years later. The Margam account could not contain the information that it does and still be written a decade or two after the discovery of the grave.
.............................................................
At Glastonbury the bones of the most famous Arthur, once King of Greater Britain, were found, hidden in a certain very ancient coffin. Two pyramids had been erected about them, in which certain letters were carved, but they could not be read because they were cut in a barbarous style and worn away. The bones were found on this occasion.
While they were digging a certain plot between the pyramids, in order to bury a certain monk who had begged and prayed the convent to be buried here, they found a certain coffin in which they saw a woman's bones with the hair still intact. When this was removed, they found another coffin below the first, containing a man's bones. This also being removed, they found a third below the first two, on which a lead cross was placed, on which was inscribed, "Here lies the famous king Arthur, buried in the isle of Avalon." For that place was once surrounded by marshes, and is called the isle of Avalon, that is "the isle of apples." For aval means, in British, an apple.
On opening the aforesaid coffin, they found the bones of the said prince, sturdy enough and large, which the monks transferred with suitable honour and much pomp into a marble tomb in their church. The first tomb was said to be that of Guinevere, wife of the same Arthur; the second, that of Mordred, his nephew; the third, that of the aforesaid prince.
Interesting.