Over the Christmas break, with nothing better to do, I did a bit more research.
ESC confirms that the roses in angles silver of this period use silver from "The West of England".
Researching silver mines in the West, there are not too many options, and of these many were not operating in the 18th century. Only two areas seem to be working: Combe Martin and Cornwall. Cornwall's main focus was on tin and copper, though some silver was produced almost as a by-product. So it does seem that the bulk, if not all the silver in this 1739 Crown would have come from Combe Martin.
A nice little addendum to a pleasing coin.