"Taylor found new evidence to suggest that a version of "Sir Aldnigar," unknown to Child, was sung in the twelfth century. If true, that would make "Sir Aldnigar" the oldest known "Child ballad."

Sorry to nitpick, Lighter, but what Taylor actually says is:

"It seems reasonable to assume that there may have been ballads by the middle of the thirteenth century and perhaps even by the middle of the twelfth." [my italics]

but:

"The tradition behind "Sir Aldingar" gives no conclusive proof that popular ballads were sung in England before the thirteenth century."

He specifically refutes the 'smoking gun' claim by Entwistle in European Balladry, based on a (mis)translation of William of Malmesbury's Gesta Regum Anglorum, that "a poem about the false accusation and surprising deliverance of Canute's daughter Gunhild was sung publicly in England in the mid-twelfth century."

Plenty of people have wanted 'Sir Aldingar', with all of its medieval touches, to represent a true ballad from that period, but no-one's nailed it yet!