Here you go Dexter, These are the words as per Henegan (I think by Merv Davy). As far as I can tell no-one seems to know of the origin of the song but these words were collected from Neil Plummer of St Stythyans, so the book says.

All the best Marion

An Rosen Wyn (The White Rose)

My agaran rosen wyn mar whek mar dek del dyfhy
An rosen whyn mar splan mar vryntyn a dhre dha gof amma dhevy.
Kensa pan whelys ow whek oll, yth esta mar dek avel ros;
Mes lemmyn mar dhys lyw dha vejeth, mar wyn avel rosen mayth os
My agaran rosen wyn, mar whek mar dek del dyfhy
An rosen wyn mar splan mar vryntyn a dhre dha gof omma dhevy