But isn't "Barbara Allen" usually in "standard English"?

Anyway, would Pepys have known a real Scotch song if he heard one?

Perhaps Mrs. Knipp/Knepp had a Scottish accent (unlikely). Or sang the song in Scottish character (more likely). Or introduced the song, based on hearsay, as "Scotch" (possible). Or perhaps that version mentioned a Scottish locality - perhaps the "North Country" instead of the West (quite possible, particularly if a Graeme was involved).

We may never know. The point is that based on what we *do* know, which is next to nothing, Pepy's informal characterization of the song as "Scotch," all by itself, has little value as evidence for anything.