I tried deleting the Cb Auto Actions plugin - this would uninstall but the error msg generated just appeared at an earlier stage of the registration procedure - I guess that one would need to explore files to locate where reference to the plugin fails.

Perhaps uninstalling would have worked in the J3 environment.

CB Auto Actions is absolutely uninstall-able. I've no idea what issue you're having here. Simply navigate to CB > Plugin Management and click the checkbox next to CB Auto Actions then click the Delete button on the toolbar at the top.

If that's not working then your database is broken as something has altered the CB Auto Actions plugin row flagging it as a core plugin. That can happen sometimes with some migration extensions (folks, you do not need a migration extension to upgrade Joomla!) where they alter the table data for whatever reason.

If issue persists please PM backend super user login credentials and will take a look.

Also Im having trouble with the install from web key - see attached Joomla is telling me the site key is wrong but Ive tried both methods and have not been able to get install from web working

The domain you're trying to use Install from Web from and the domain in your site key don't match. Ensure live_site in configuration.php is empty, access your site like you normally would, go to CB > Configuration > Integrations, empty the "Install from Web Site Key" parameter, then click the button to get a site key and it will pre-fill the domain for you.