The importance of this sort of evidence shouldn't be underestimated. As Jim says this sort of song rarely has entered oral tradition in England, or perhaps that is what the collectors led us to believe. In Scotland less so, and in Ireland if you look at collections they are quite common. Off hand the only English example I can think of is the song about the Nottingham and the Mars skirmish by Phil Suamarez in 1746 which morphed into a whole series of well-known sea songs. And perhaps some of the local hunting songs still being made. I followed Rachel's link but no date is given. James/Rachel, can you put a date on it for us please?
Jim,James, will you be at Liverpool on Saturday?