Jon says, “Very Sussex this. Sung by Shirley Collins on the brilliant Voices album on Fellside.”
Sussex through and through as appropriately enough it’s in the Copper archive, collected by Bob from a Harold “Jim” Swain who got it from a shepherd near Felpham. Hastings’ born Shirley Collins originally recorded this for her Adieu To Old England and she described it as “a little confused,” but was suitably taken with it to record it as she found it. This seems to have several variant takes on Mossom, Mossen, Mossing, etc, although none of them actually seem to have much meaning for the word. There is, however, a River Mosson in France and the trade back and forth between Southern England and our near neighbour may account for a cross-channel origin, although that’s pure speculation. Mainly Norfolk has Shirley’s recordings noted and you can also link to Mudact here although it doesn’t really add much.