I have finally seen a copy of this in print, only 3 verses, ('our' first three). This was in a small song book entitled "Canwn!" (We Sing ?) by a publisher called "Y Lolfa" (The lounge ?).
Whilst claiming the copyright on a collection of 102 songs, they seem very slow at admitting anyone elses copyright. It may be that all the songs are out of copyright, but I doubt it.

Nigel