by 1800 (when the kind of tune books that Richard Robinson was talking about were being written out) there seems to have been a cohort of village musicians who were not members of the educated middle classes but could nonetheless read and notate sheet music

The tunebooks from the fifers of the Black Watch that I transcribed and put on the flute pages on my website are an interesting example. They were presumably young soldiers from Argyll, training near Oban on their way to fight in the Napoleonic Wars. Their tunebooks are competently written, as far as the notation goes - a few boobs and discrepancies but nothing to get in the way of practical usability. But their handwriting was semiliterate, and most of them used the covers to practice signing their names. So it didn't take a high educational level to become a user of music notation.