Sorted! I cleared my cart & logged out & back in to make sure I had the right password for you & had another go & it worked ok. My first attempt to add ES I wasn't logged in & the XF licence was also added. Seems you can't then stop it being added if you then log in - no matter how many times you remove from cart & try again. I added it again just now while I was logged out, then logged in & got caught in the same loop. Cart cookie issue?
Thanks anyway!