Sian - very interesting what you say about Plethyn. I enjoy their sound and wish there were others singing in that style. Is there a way to quantify what the plygian style is - 3rds, 5ths, no embellishments, no call and response?
The new 101 Carolau cd has a good variety of plygain on one disk 101 o Garolau   
Dydd gwyl ddewi sant hapus, Chris