And this is for you Steve because you have referred to the possibility of some earlier lost ballad more than once. This is the whole excerpt from "Christopher" which deals with LL:

And she would sing to him. Yes, first " A little ship was on the sea, it was a pretty sight," and then " Lord Lovel and Lady Ancebel." Yes, the whole of it — even if he was asleep before the last verse. . . . A voice singing softly — singing, as it were, under its breath, and for Christopher alone — was heard then in the night nursery, chasing fears away, and spreading a gentle calm that lapped him round like tiny waves of the sea. *'

' Oh, that's a long time, Lord Lovel,' she said,
To leave a fair lady alone ' "

The voice faltered. Christopher felt the hand removed from his head for a moment. " Mummy ? " " Yes, darling."

"I'm not asleep — quite."

The hand went back. It held something then which had been fumbled for without any actual break in the singing. Christopher, with eyes tight closed, wondered why. But the handkerchief seemed like part of the hand, and he felt quite safe and sighed contentedly.

And so it is Lady Ancebel,
But I must needs be going.'"

The voice, threading the verses on a slender string of melody, grew further and further off. Christopher heard about the milk-white steed, and "Adown, adown, adown, adown," and was conscious of the approach of the line which to Mrs. Herrick always seemed to have too many feet. At " a branch of sweetbriar," he tried, with a vague intention of announcing that he was still awake, to say Mummy once more, but the word would not come, and the last verse of all mingled itself with new and happy dreams :

"They grew till they grew to the top of the church,
And when they could grow no higher
They grew into a true lover's knot,
And so they were joined together."

Collecting songs is like fishing. Certain ones are caught but there's more in the sea as they say. Maybe this passage represents a version that was never collected.

Adown, adown, adown, adown.

Obviously the refrain. It's different from other versions we know of. How interesting. Note all the references to the sea also. And don't forget the true lover's knot is a sailor thing to begin with. Maybe LL is just seafaring ballad that eventually became a lullaby in each harbor town that embraced it. This novel begins with Christopher's birth on a ship rocking away in the sea :-)