That's the lot of it, I'm afraid. We get this from a chronicler's Historia Eliensis (12th century): King Cnut "composed in English a ballad (cantilenam), which begins [note!] as follows:
'Merie sungen the muneches binnen Ely,
Tha Cnut ching rew ther by.

Roweth, cnihtes, noer the land,
And here we thes muneches sang.'"
-- which is then turned into Latin, and the chronicler adds"and so the rest, as it is sung in these days by the people in their dances, and handed down as proverbial."
[quae usque hodie in choris publice cantantur et in proverbia memorantur]
i.e.it was a common pop song of the time, and so maybe he thought he didn't have to give all the words.
My references are Francis B. Gummere, The Popular Ballad (1907), rep. Dover 1959, pp. 58-59.
-and G. H. Gerould, The Ballad of Tradition (1932; 1957), pp. 195-6.