I have written about, and given evidence for, this "addoliad trôns" before.... notably here...
http://therockyroadtowelsh.weebly.com/blog/trons
and there's a link to yet another thing about it on that !
Meanwhile , here's an interesting take on this seminal song ( no pun intended, subtle though it may be) ....
Also ... is there another Welsh band which routinely features the double bass?