"===I believe Scotch was historically an English form, which only found general use within Scotland during the 19th century. ===

I think not, Dave Mackenzie. See Boswell ~~ he was conversing with Dr Johnson admittedly; but he was a Scot himself! "

I said general use. Boswell was one person, and first met Dr Johnson three years after he ran away to London in 1760, and was by then an English resident.