I re-read the footnotes in Prof Bonner's 'Pirate Laureate'1947, where he mentioned "Venture Gwen, or the Plaint of the Widow" (Mentra Gwen, neu Cwynfau y Wraig Weddw, and found Bonner did in fact refer to the exact source Sian came up with: "Journal of Welsh Folklore Society (1930), III, 45."
However Bonner adds
"See also Journal of Welsh Folklore Society II (1914-25), 122."
So perhaps there's something in volume II that would benefit this resarch too.