I've just come across this in David C. Fowler's "A Literary History of the Popular Ballad", quoting the first stanza of a version in William Shenstone's "Miscellany" (1759-63):

Giles Collin came home unto his mother,
O Mother come bind my head
For before eight o'clock in the morning
O Mother, I shall be dead.

Fowler argues that this opening suggests that "Giles Collin" is a continuation of "Clerk Colvill", which must therefore have been in existence in some form at least six years before it appeared in David Herd's book in 1769, but he continues: "A date much earlier than this, however, seems precluded by the pseudo-archaic and imitative style of the ballad as we know it."