Whilst admitting my lack of knowledge of pre-1500 balladry the sceptic
in me points out I have not come across any notable evidence for the type of ballad that constitutes the bulk of Child's anthology pre-1500. What little there is is very scant, not just in Britain, but the rest of Europe. The subject matter is often earlier but there is no evidence to show that a ballad was written contemporary with the earlier events. There seems to be a big gap between the writing of the last sagas and romances, and the appearance of the first ballads.
Even so it would be great surprise if some of the content of the sagas and romances had not been incorporated into the later ballads. I have seen it expressed somewhere that the ballads predated the sagas, but I find this difficult to swallow. Most of the older books I've read present the theory that the European ballad style evolved in France from the carols which would seem reasonable to a layman like me.