It is a rash and foolhardy endeavour to attempt to date a story, or even trace its journey. As most folk tales, by their very nature, are oral in nature and not all get recorded in written form (still less in other recorded media), and as a component of many tales is to claim some form of authority for it (as in: I heard it from my father's father/found it in an old book/always heard it told in this village)it is very difficult to even attempt to sort out origins and derivations. I've been dealing with this issue repeatedly with my PhD research. However, my examples were not intended to suggest a dating or an origin for the stories, simply to point out that they were indeed recorded in written form far earlier than I might have expected. Just how the "fairy midwife" story might have started life, and where, will remain a mystery. Yes indeed, there will be superstitions around ships becalmed for no apparent reason, but again this particular iteration of the story is recorded in a different context to where I have heard it before (as in: this is more of a historic chronicle and not a ballad). It fuels my curiosity - did these stories spontaneously evolve in similar form in different geographic areas, or were they carried by traders, or family members, or some other way and then rooted themselves somewhere else, to be told as local stories in their new homes?
In the case of the Fairy Midwife story, Gervase of Tilbury (who wrote it down in the mid to late 12th century) travelled extensively in Europe, starting with the court of Henry II in England and France, and then in Sicily at the court of William II, and then in Arles, attached to the Archbishop's palace, where he was known to entertain Alfonso II, king of Aragon. People did indeed travel, rather more than we might think. However what intrigues me most is not that people travel, but that people then locate the story in a new home, so this story is told as if it belongs to France, or Wales, or Ireland, or Scotland, or England - or any number of places.
And thank you, Pseudonymous, for your kind comments.