Source: Edith Rickert, "Ancient English Christmas Carols: 1400-1700," Chatto & Windus, 1914, p. 129.

LYR. ADD: A, A, A, A, NUNC GAUDET ECCLESIA

1.
Listeneth, lordlings, both great and small,
I shall tell you a wonder tale,
How Holy Church was brought in bale,
Cum magna injuria.
2.
The greatest clerk in all this land,
Thomas of cantebury, I understand,
Slain he was by wicked hand.
Demonis potencia.
(By the power of the devil)
3.
Knights camen fro Henry king,
Wicked men without leasing (i. e., truly)
There they diden a wonder thing,
Ferventes insania.
(Raging in their madness)
4.
They sought him all about,
Within the palace and without;
Of Jesu Christ they had no doubt, (fear)
In sua malicia.
(In their madness)
5.
They opened their mouths wonder wide,
To Thomas they spacken mickle pride,
"Traitor, here, thou shalt abide,
Ferens mortis tedia."
(Bearing the burden of death).
6.
Thomas answered with milde cheer,
"If ye will me slay in this manner,
Let them pass, all that are here,
Sine contumel(i)a."
(Without disgrace).
7.
Before his altar he kneeled adown,
There they began to pare his crown;
They stirred the braines up so down, (upside down)
Optans celi gaudia.
(Hoping for the joys of heaven).
8.
The tormentors about (they) start
With deadly wounds they gam him hurt;
Thomas died in Mother-Church,
Pergens ad celestia.
(Attaining to heaven).
9,
Mother, clerk, widow, and wife,
Worship ye Thomas in all your life;
For fifty-two points he lost his life, (The Constitution of Clarendon, 1164)
Contra regens consilia
(Contrary to the king's counsels).

This version is a bit more readable.
Not my idea of a carol; 'political protest song'- I see your point. The website below doesn't mention the original source. An article by Louise Pound suggests minstrel songs, improvisations of peasant communes, etc. Most seem to have been designed for recitation. Little is known about them. I don't have the article, from PMLA vol. 35, no. 2, 1920 (Pub. Modern Language Assn. of America). There should be some information on the web, or perhaps McGrath will comment.


From "The Hymns and Carols of Christmas." www.hymnsandcarols0fchristmas.com/Hymns_and_Carols/listeneth_lordlings_both_great_an.htm