Just leaving these notes for next time rather than search the interwebs again.
- At the boot options screen select the Advanced Ubuntu item, then the recovery item
- At the menu that appears select root command line
- mount -o remount,ro
- fsck.ext4 -y /dev/mapper/appserver–vg-root
- resume boot
I needed to reboot again to get the boot file system check to perform and get back to read write mode. Still not sure how it got into this state in the first place though ..