Tried again.
I edited GRUB to boot with nomodeset - this didnt fix the login screen.
Them in a TTY I install nvidia-prime then ran
sudo prime-select intel
After a reboot graphical logon screen (lightdm slick-greeter) and logged in successfully to the desktop.
So the regression here is that nvidia-390 is not installing the same config as generated by prime-select
Tried again.
I edited GRUB to boot with nomodeset - this didnt fix the login screen.
Them in a TTY I install nvidia-prime then ran
sudo prime-select intel
After a reboot graphical logon screen (lightdm slick-greeter) and logged in successfully to the desktop.
So the regression here is that nvidia-390 is not installing the same config as generated by prime-select