John Paddy Browne includes the song in FOLK SONGS OF OLD HAMPSHIRE (Milestone Publications, 1987); this is the sole reference to the song in Robert Waltz's Ballad Index.

Browne's book intersperses old and new songs he regards as native to the region; the "old" songs (varying in antiquity from Henry V's Conquest of France to Buttercup Joe) are given without any attribution, while the songs written by Browne's contemporaries are explicitly cited . . . except for this one. Browne does cite A.L. Lloyd several times elsewhere in the volume, including his own conversations with Lloyd about the nature/definition of folksong. If the song's attribution to Lloyd elsewhere in this thread are accurate, presumably he passed it off to Browne, and Browne (for whatever reason) seems to have regarded it as traditional.

But I can't imagine that is. Even if Lloyd truly rearranged this from a broadside ballad, exactly what broadside was it? I can't identify a likely candidate after a quick perusal through the Bodleian's online archive, and a fair chunk of the text (e.g., "flatten their mountains / "flow like fountains") seems to belong to Lloyd's poetic voice.

--
Mars for Evermore, as given by John Paddy Browne, 1987:

Well now, brave boys, we’re off to the main,
Roar Agamemnon, roar,
To load our ships with the dollars of Spain,
Mars for evermore!

They tell us thirty ships of the line
Roar Agamemnon, roar,
From France and Spain on the sea do shine,
Mars for evermore!

Those ships of France and Spain my [sic] shine,
Roar Agamemnon, roar,
But they’ll not forget the year ‘05,
Mars for evermore!

The guns did rattle and the shot did hail,
Roar Agamemnon, roar,
And every ship fought fire and flame,
Mars for evermore!

The streams of blood from their scuppers did flow,
Roar Agamemnon, roar,
The blue sea ran with purple gore,
Mars for evermore!

We’ll burn their boats and flatten their mountains,
Roar Agamemnon, roar,
We’ll make their blood to flow like fountains,
Mars for evermore!

Well from our guns blew the British thunder,
Roar Agamemnon, roar,
And that’s how we keep our enemies under,
Mars for evermore!