A counsel of perfection? Of course! Is anything less than that worth aspiring to? Seriously, though, it doesn't require musical training (I have none, though it would have been useful) to set down a normal song tune by ear, just patience and practice; and some midi-writing software that will play the melody back to you as you write it, of course...
I've been down to the University to consult Bronson, and alas, MacColl's family variant was too late to make it into the book (though I was in a bit of a rush and neglected to check for addenda in the final volume). Neither was the Central Library any help; the MacColl ballad recordings they have are from other series and don't include this one. What I can report, however, is that there are two fragmentary texts in Bronson's group B which contain the refrain you quoted; both are from Greig and Keith's Last Leaves of Traditional Ballads and Ballad Airs (1925). #43:12, The Broomfield Hill, was sung by a Mrs. Gillespie, and runs:
Leddrin thee thoo an' a',
Sma' an' nanny hue, O,
Come sell a me my ebrachee,
An' a maiden I see you, O.
#43:13, Leatherum thee thou and a', was sung by Alexander Robb of New Deer, 1925:
There was a knicht an' a lady bricht,
Set trysts amang the broom,
The ane to come at twelve o'clock,
An' the ither true at noon.
Leatherum thee thou an' a'
Madam aye wi' you,
An' the seal o' me be abrachee,
Fair maiden I'm for you.
The third of the group is from Lady John Scott's J.K. Sharpe MS.; no text was recorded for it.
All Bronson has to say by way of textual comment is "The second group... affects a burden with nonsense syllables..."
I'm rather disinclined to think there's any magical sense to these words; on the face of it leddrin/ leatherum and abrachee/ abrachee look like old dialect (or even, at a stretch, garbled bits of Gaelic, though that might not be so likely in the North West), the sense of which was no longer known by the singers themselves.
So far as the three tunes go, Bronson mentions that other variants are also found attached to #231, The Earl of Errol and #274, Our Goodman. It's worth mentioning that Martin Carthy (who has used Last Leaves as source for a number of songs) used #43:12 for his re-working of Lang Johnny Mor, and #43:13 for Prince Heathen.
In case it's of any help, here are three midis, available for now via the South Riding Folk Network site; is any of them at all like the one MacColl used?
#43:12 The Broomfield Hill
#43:13 Leatherum thee thou and a'
#43:14 The Wager