Check if your account is Steam. I have since uninstalled and reinstalled through Steam and rebought Heavensward through Steam. Now installing Heavensward (which is taking longer than it did to d/l on Friday lol)

The key has worked too, so Heavensward will be available after this reinstall. It's long-winded, and I'll have to chase SE Europe for the initial refund, but it'll all work out in the end.

It could also have been easier, had SE informed people who initially had FFXIV:RR outside of Steam, that their Heavensward addition would have to be bought through Steam IF their account was registered as Steam anyway, but the delay of half the Pre release is bearable >_>