I'm sorry Malcolm, You're quite right about the ascriptions being only anecdotal. I just get a bit silly sometimes. The internet is already overflowing with crazy stuff you can't trust by silly people.
What I want, in fact, is not silly stuff but trustworthy dates.
SO
I went back to what started all this for me: W.H. Bonner's "Pirate Laureate" p.102 to revise what he wrote (he was professor of English at the University of Buffalo in 1947):
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"Students of folk song now recognize the metrical pattern as a variant of a well-defined old Welsh ballad known as "Venture Gwen, or the Plaint of the Widow" (Mentra Gwen, neu Cwynfau y Wraig Weddw).(18)
........When the notes of "Captain Kidd" were first set down they were ascribed to "Nicholson"; but such ascriptions in the old song books are misleading, Professor George Pullen Jackson informs me, being usually the names merely of those who set notes and provided harmonic parts for the oral tunes. On the other hand, there is reason to think that "Captain Kidd" is very old. Miss Anne Gilchrist, a thorough student of English folk song, traces it back through an old Scottish ballad, "My Luve's in Germanie, send him hame, send him hame," to the sixteenth century.(19)"
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Footnote 18: Journal of Welsh Folklore Society (1930), III, 45.
See also Journal of Welsh Folklore Society II (1914-25), 122.

Footnote 19: G. P. Jackson, Down-East Spirituals (New York, 1943), 259—61. For further comment, see:
J. A. Lomax, American Ballads and Folk Songs (1935), xxxviii—xxxix.
Eckstorm and Smyth, Minstrelsy of Maine (1927), 247—49.
C. J. Finger, Frontier Ballads (1927), 29—32.
W. B. Whall, Sea Songs and Shanties (1920), ix.

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What I wondered was: did Anne Gilchrist trace it back to the sixteenth century in a detailed way? If so, where did she publish it? Because I want to put "C16th" on something too. It turns out that the footnotes 18 and 19 are a bit confused because 'Sian, west wales' tipped me off that a Miss A.G. Gilchrist wrote the following about 'Mentra Gwen' in the Journal of Welsh Folklore Society 1930), III, 45:
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"This sounds to me like a Scottish tune. It belongs to a characteristic verse-metre, the (probably) earliest known example of which is suggested by a title in 'The Complaynt of Scotland,' 1549.'My lufe is Iyand seik, send him joy, send him joy.' This may have suggested Hector Macneill's: 'My luves in Germanie, Send him hame, send him hame.' There were also songs, of probably earlier date than the above, on Captain Kidd, Admiral Benbow, and the notorious criminal Sam (or Jack) Hall; and in the nineteenth century a revival hymn in 'Richard Weaver's Tune Book' 1861, was modelled upon the same stanza. All these tunes, together with the Welsh air, appear to be variants of the same original. 'Captain Kidd' and 'Sam Hall' are
the dying confessions of villains, and the revival hymn is obviously suggested by one of them. It begins: 'come ye that fear the Lord Unto me, unto me, etc.' A verse of 'Captain Kidd,' with a somewhat modernized American form of the tune, may be quoted from an American collection, 'Our Familiar Songs, and Those Who made Them' 1889 (see below). Captain Kidd was hanged in Execution Dock on the Thames, in 1701, and the ballad is probably contemporary.
- Miss A.G. Gilchrist"

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Assuming that 'Miss A.G. Gilchrist' and 'Miss Anne Gilchrist' are the same person: could this paragraph be all that Bonner was referring to as "tracing it back...to the sixteenth century?" Is this paragraph in the Journal the full extent of her discourse? Or did she write a great thesis on it elsewhere that will give me full knowlege and the right to type "C16th" next to 'Germanie Thomas'? Could her thesis be in another of the books in footnote nineteen?
- Or is this as far as this trail goes?