While I have as yet been unable to find lyrics from a primary or secondary source, the following lyrics (or something like them; my ability to transcribe lyrics from tape is severely impaired) are sung by the Camerata of London on their CD Popular Music from the Time of Queen Elizabeth I (Saga):

All in a garden green, two lovers sat at ease,
As they could scarce be seen above the leafy trees.
They lovèd lofty full, and no wronger than truly,
In the time of the year cam betwixt May and July.

Quoth he, "Most lovely maid, my troth shall ay endure,
And be not thou afraid, but rest thee still secure
That I will love thee, long as life in me shall last:
Now I am strong and young and when my youth is past.

"When I am grey and old, and then must stoop to age,
I'le love thee twenty-fold, my troth I here engage.
My love shall be the same, it never shall decay,
But shine without all blame, though body turn to clay."

She listen'd to his song, and heard it with a smile,
And innocent as young, she dreamèd not of guile.
No guile was meant by Will, for he was true as steel,
As was there aught deceit when she made him a will.

Full soon both two were wed, and these most faithful lo/vers
Base are but born and bred, example to all others.
They lovèd lofty full, and no wronger than truly,
In the time of the year cam betwixt May and July.

(That's my best attempt at transcription; if you have corrections, or another version, please email me.)

Note: the first half of the final line must be repeated in order to make the lyrics fit; the tune is also slightly altered to make the syllables more natural.