Ubuntu GNOME 17.04 won’t start

I’m running Ubuntu GNOME 17.04 on VirtualBox with an image from osboxes.org

Updated last night. Today, it won’t start. It hangs at various points during boot up, with messages such as

A start job is running for Daily apt activities ( 55s / no limit )

[ OK ] Created slice system-getty.slice

Started User Manager for UID 120
Stopping User Manager for UID 120
Stopped User Manager for UID 120
Removed slice User Slice of gdm 

What’s the best way to proceed? Obviously I could just delete the VM and create another but it could well just do the same again.