My Parallels Desktop 8 serial isn't working when trying to upgrade to version 11

So, I've had version 8 for a few years now. I just paid the £35 for the upgrade to 11. I know I'm allowed to upgrade, as stated here:

Important: Upgrades are possible for Parallels Desktop 8 and later. For activation instructions with a Parallels Desktop 10 upgrade license from Parallels Desktop 8 or 9, see article #122648.
​

When I go to install the latest version, my new version 11 key is fine (I get the green tick), when it asked for my previous version key... I get nothing. No tick, no cross, nothing. The only option available to me is the start a trial version.

Am I missing something?

Thanks,

Mark.

 

Hello MarkN2,
Please check your private message.