Jon,
Looks like my memory failed me. Most of the references that can be dated in the other 22 garlands hit on 1759, but Duke William is mentioned and he didn't become duke till 1764 so 1765 would be a good guess.

A song sung by Mr Lowe at Vauxhall doesn't help much as he was singing at Vauxhall from at least 1750 to 1780. Otherwise we have the Battle of Minden 1759, and Admiral Hawke, Boscowen and Belleisle
all get a mention pointing at about 1759.

The only ballad I had time to copy down was a version of The Little Dun Mare' which at least helps to date that.