Ubuntu VM starts up with black screen after upgrade

XCP-NG running on x64 hardware. Problem with Ubuntu VMs with a graphical interface. Upgraded them from v20.04 to v22.04 and now when they start up the screen ends up black. The VMs genuinely start. Watching the boot process it all seems normal (so you see the Ubuntu circular logo etc) - but goes black just at the end.

Seems to be a known issue - but only fixes I could see were to recreate the VM from the ISO, or to have the Grub menu display to allow you to select an earlier version.

Any wisdom greatly appreciated. Thanks.