Solved! Unearthed this solution thanks to the pcraig3 advice to manually edit the database https://wordpress.org/support/topic/multisite-dashboard-link?replies=6
Please note that you need to make sure you are editing the url for your new site. If you are getting the problem I outline above you go to the wp_2x0somenumbers_2_options table rather than the options table further down that will not have the _2_ (or whatever your new site id is). You will see the name/tagline for your new site in that table if you have added it but it will have the url of your main site, not your new site. That’s what you change.
thanks,
Will
