Once more I had to change my address, Now I am www.goldenaer.com
I am still getting my ebook ready with answers to everything !! but meanwhile, here's my sources. Note there isn't an exact tune, the expression is very variable depending on your instrument: harp song or fiddle. For more, ask the Welsh Folk Song Society https://canugwerin.com/en/english-fp/

"Mentra Gwen, nea Cwynfan y Wraig Weddw" in article by J. Parry (Bardd Alaw)'s Welsh Harper Vol.II No.115, The Journal of Welsh Folksong Vol.II, 1919, part II, page 122. NO.74

"Mentra Gwen (Plygian Carol)" in Article by Phylis Kinney on Plygian Carol tunes. No.6 MS 2254. Welsh Folksong Society Journal, Canu Gwerin (Folk Song) No.11 1988 pp 27-28. Cymdeithas Alawon Gwerin Cymru, ISBN 0 9510307 5.   

Two variations, "Mentra Gwen (Fiddler's Tunebook 1752)" and "Mentra Gwen (British Harmony No.32" both in an Article by Cass Meurig's "Singing to Fiddle playing in the 18th Century" in Canu Gwerin (Folk Song) No.24 2001 pp 20-41, Cymdeithas Alawon Gwerin Cymru, ISBN 0967-0559