Bingo - that fixed it. Thanks.
Pretty sure I activated it and did a restart before I entered the plugin tweak. Then I did the plugin tweak and another restart, but I don't recall checking the activate.
Its not entirely clear to me why both the drop down in the plugin configuration, and the plugin tweak are needed - apart from the fact that the drop down doesn't include all possible formats.
Added - the Save to Disk flyout entries now have 'CBZ' when I'm in my Media Library, and 'EPUB' when I'm in my Main library etc
