EDIT (KB just updated today): https://support.frontier.co.uk/kb/faq.php?id=325&q=Product%20Access%20Code
Update #1: Okay, we're good thanx to the KB ... please disregard--er, oh no ...
Update #2: Ugh, I've managed to invalidate my order by some cross-site scripting that my browser blocked. It's not listed in my orders, but the code no longer works when I try to check out (now invalid). Removing from the cart, logging out, logging back in and trying to add it again doesn't resolve the issue. I honestly give up. I feel for Frontier, as they have to partner/outsource portions of the store. So I really cannot complain.