"The material and style will show varying influences from outside the tradition"
Not as many as you might imagine Vic.
Most of the singers we questioned, Traveller or settled, insisted that there was a "right" way to sing a song, and once you strayed from that you were "making a mess of the songs".
A rather memorable occasion was when we witnessed a discussion on the differeing styles of two brothers, one who we recorded at length.
Both had learned their songs from their parents, primarily their father
One sand in straight traditional style, the other preferred Country and Western, and sang his songs in this manner, so you got 'The Outlandish Knight', 'The Grey Cock' and 'The Rambler From Clare' like 'cowboy' songs
The group of Travellers we were with overwhelmingly said that 'Little Bill's' straight style of singing was the 'correct' one and Andy's "messed about with the story"   
Despite pressure to do so, we refused to offer a opinion, though, in my opinion, acculturation had taken place; nothing to do with a development of styles, but one way of singing had actually killed the original style and changed its utterance - i.e. removed the narrative nature of the songs - every singer we questioned described themselves (in their own words) as storytellers whose stories happened to come with tunes.
MacColl makes the point earlier in The Song Carriers in his comments on Anthony Newley's 'Strawberry Fair'
Jim Carroll
"Is it animal, mineral or vegetable? There are those who consider it to be folk music. It certainly began life as a folk song. Both the words and the tune were conceived in the folk idiom and it has been sung by generations of folk singers. And yet, there are many people who would deny that it is still a folk song when performed in that particular manner. What then, has happened to it? Its utterance has been translated, its idiom changed to that of pop-music It is as if we were to take over a pop song and recast it in a classical mold, and then have it performed by a string orchestra whose natural metier was, say, the Beethoven quartets. Do you think it would still be pop music? Conversely, if we took one of those same quartets and performed it on three electric guitars and bongo drums, would it still be Beethoven? It would not. The imposition of styles and idioms foreign to a particular form results in that form being transformed. It becomes something different. Not necessarily something worse or better, just different".