Unable to log in due to session cookies not being passed in
For anyone else who stumbles across this problem I did finally manage to solve it with the following which ensures the credentials are pass for each call consistently. I'm assuming where CORS adds in the additional OPTIONS request, the credentials we're going through.

jQuery.support.cors = true;
    jQuery.ajaxSetup({
        xhrFields: {
            withCredentials: true   // Pass session cookie with requests
        }
    });