Hi Graham,
If you check your serial number here (https://secure.f5.com/validate/validate.jsp) does it show an active support contract? If you want to run 11.2.1 you'd need to have had support on the release date of 2012-08-20:
sol7727: License activation may be required prior to software upgrade
https://support.f5.com/kb/en-us/solutions/public/7000/700/sol7727.html
If you did have support after that date, you could try re-activating the license on the slot you're upgrading from and then retry the upgrade. This is the best practice process. But generally, it should work if you re-activate the license after the upgrade too.
Aaron