After turning this on and being happy with myself for finding a simple solution for SSL I created a new site.
The Visit Dashboard link that appears now links to https://wp-admin directly which is clearly a wrong link.
If I go to network admin > sites and hover over the Dashboard link I have a long url that again goes nowhere:
https://mainsite.org/wp-admin/network/default.mainsite.orgwp-admin/
With the plugin turned off it refers to the right link:
https://default.mainsite.org/wp-admin/
Clearly this is a plugin error.
Any tips on fixing this or is this plugin just not workable with Multisite?