I've found the text I had in mind when I started this thread, it is by Alan Lomax and Peter Kennedy, 1960 and 1999, on the booklet that goes along the two Caedmon/Topic LPS, now Rounder CDs, dedicated to the Child Ballads:

(…) Many of the 300-odd narrative pieces canonized by Professor Child (called familiarly today the "Child ballads") have long since passed out of oral circulation. One hundred and forty, in full or fragmentary form, have been discovered in North America, whence they were brought by British emigrants. Our research, done during the past decade, has unearthed fifty still in circulation in Great Britain, most of which we reproduce in these two recordings. (…)
A few (ballads), among them "Young Hunting," "Fair Margaret," "The Wife Wrapped in the Wether's Skin," "The Wife of Usher's Well", that are quite common in America have not yet turned up in Britain. "The House Carpenter" and "The Farmer's Curst Wife" were found very frequently in America and much less so in Great Britain, versions of "The Gypsy Laddie" and "The Trooper and the Maid" being far better known. Certain ballads seem to be better remembered in Britain than in the United States, among them: "The False Knight Upon the Road," "Broomfield Wager," "The Baffled Knight," "The Braes o' Yarrow," "The Jolly Beggar," "Andrew Lammie," and "The Laird o'Drum." Until we started our work no version of "The False Knight" had been collected in Britain, but like "Edward," versions turned up among the tinkers and Gypsies.
By far our best sources have been the tinker singers of northeast Scotland, who have given us full versions of certain ballads that rarely occur elsewhere, since they are of local interest or of special relevance to their lives. In fact, it appears that tinkers, Travelers, and Gypsies have recently played the principal role in the transmission of the Child ballads in the British Isles. Around their campfires the ballads are sung and ancient Gaelic legends are told today as they were centuries ago. The stamp of tinker interest shows up in the popularity of such songs as "The Jolly Beggar," "The Trooper and the Maid," and "The Gypsy Laddie." (…)

The contributions to this thread have added many more knowledge, for example on the significance of the Irish recordings of these ballads. I'd be interested in knowing your opinion on the notes by Lomax and Kennedy. Thanks again. R