Ive solved it and I wanted to share the solution with the community because its stupidly buried....
You need to goto "Samsung Link" web page, not your samsung account and login with your normal samsung account user and pass you set up for your phone/Tab.
I cant quite remember how I got this, but you need get too Samsung apps and you should get a "my device" and a picture of your device on the top left. Click on the very small settings cog next to the words "main device".
Click the end tab "my device", scroll to the bottom of the list and click "Modify" and just uncheck every device youve sold or dont use and click "Update".. and there you go.. all your slots back that your not using.
I hope that helps someone.. but like i say... you may have to dig through the site a little,.. to find the "my device" page.