Sooooo, this is a might awkward.
I added the import line manually as you suggested and it underlined with the error "The import net.minecraftforge.common.config.Config conflicts with a type defined in the same file". After staring at it for a moment I suddenly realised maybe it didn't like my config class being called "Config", sure enough a quick rename and the annotation immediately imported ?
So thanks for the response, while not quite how intended it still solved the problem.