Possibly worth at this stage quoting the notes accompanying the version in "Carols for Choirs" by the Oxford University Press (ISBN 0-19-353222-0)
"The tune, and the words of verses 1, 2, and 5, are as sung at Queen's College, Oxford. To provide an optional extension of the Oxford carol, two verses have been incorporated (nos. 3 & 4), fitted to the Oxford line, from a Boar's Head carol of BM MS Addit. 5665, c. 1500, bearing the signature Smert, and probably from the West Country, where Richard Smert was rector of Plymtree near Exeter, 1435- 1477."
The 3 verses claimed as being from the original are:

BOAR'S HEAD CAROL

The boar's head in hand bear I
Bedecked with bays and rosemary
I pray you, my masters, be merry
Quot estis in convivio (so many as are in the feast)

CHORUS: Caput apri defero, Reddens laudes domino (the boar's head I bring, giving praises to God)

The boar's head, as I understand,
Is the rarest dish in all this land,
Which thus bedecked with a gay garland
Let us servire cantico. (serve with a song)

Our steward hath provided this
In honor of the King of bliss
Which, on this day to be served is
In Reginensi atrio: (in the Queen's hall)

Nigel