We have the plugin working without SEF. However, we can't get it working with SEF.
With SEF enabled the user attempts to register and on the linked In "Allow' button it seems to freeze or go into a redirection loop.
Any ideas how we can resolve this please?
Are you using Joomla's built-in SEF, or a 3rd party extension? JLinked is used on many sites with SEF, so it should definitely work. It's possible there's some special configuration that may be needed for the SEF extension you are using.