In Mike O'Connors book St Petroc's Land, it states: English words and tune collected by Baring Gould and Fleetwood Sheppard from James Olver of Launceston (SBG MSS vol 2 162) Cornish words noted by Edward Chirgwin in the Gwavas Mss of 1698.
The notes on the song at the bottom of the page add..... Some think that the tune, unique to Cornwall, comes from the time when Cornish was spoken. The Cornish words were written out by Edward Chirgwin so that the song's rhythm was clear, As the verses are close in meaning to Olver's, it is reasonable to connect his tune with those lyrics. This is one of the few songs with original Cornish lyrics. Late Cornish spelling is also retained.

The words given in late Cornish are:

Pelea era why moaz, moes fettow teag,
Gen ackas pedden dew
ha ackas blew mellen
Moas than ventan, sarra weage,
Rag delkiow seue gwra moesse teag

Ra ve moas genba why, moes fefettow teag
Gen ackas pedden dew
ha ackas blew mellen
Grew mena why, sara weag
Rag delkiow seue gwra moesse teag

Fatla gwra ve ackas gorra why en doar
Gen ackas pedden dew
ha ackas blew mellen
Me veddn sefuall arta, sara weage
Rag delkiow seue gwra moesse teag

Fatla gwera v ackas dry why gen flo?
Gen ackas pedden dew
ha ackas blew mellen
Me veden e thone sarra wheage
Rag delkiow seue gwra moesse teag

Pew veda why gowas rag seera rag as flo?
Gen ackas pedden dew
ha ackas blew mellen
Why ra bose ye seera, sarra weage.
Rag delkiow seue gwra moesse teag

Pendre vedda why geal rag lednow rag as flo?
Gen ackas pedden dew
ha ackas blew mellen
E serra veath trehar, sarra weage.
Rag delkiow seue gwra moesse teag

Don't know if this helps or what you have already, but there you go!"
Cheers, Lucy