I’ve tracked it down to a problem with resolution.ini
If I delete resolution.ini and quickly reboot the box before it gets rewritten then the box boots up fine in 1080p mode.
If I then change Kodi’s GUI to 2160p and reboot the box resolution.ini gets changed to hdmimode=2160p60hz and the box just hangs at a 4K version of the CoreELEC logo and I have to delete resolution.ini to get it to reboot normaly.