I found the cause - the boot loader was executing the SUSE environment which was 4.8 instead of the Photon one.
By manually changing the boot to Photon - 8.1.1 successfully booted.
Changing the boot loader to use Photon as the default via Yast2 corrected the issue and the VM now reboots into Photon and 8.1.1.
Perhaps this is a bug in the upgrade path ?