Aha!

On taking a closer look at the record of Welsh Folk Dance Music. I see that the particular track does mention "Nos Galan, Glanbargoed, Llwytcoed" That suggests to me that the tune is "Nos Galan" and that it was collected at Glanbargoed, and that "Dawns Flodau Nantgarw" is the name of the dance and not the tune.

Mystery solved?

My thanks to Sian,West Wales, for putting me on the trail.