I'm pretty sure it's "Lliw Gwyn Rhosyn yr Haf". The scansion works. It's most recently been published in Can y Werin (www.gwynn.co.uk) with a new English translation by Gwynn Thomas. It seems to me that it might be in that old collection that Boosey & Hawkes published years and year ago - and I can't find my copy. It was popular with the North American Welsh so that may be where your correspondent learned it. On 'googling' it, it seems that Gustav Holst might have arranged it for choirs too.

sian