How can I get past the blank purple screen on my Ubuntu 18.04 LTS system?

[マスターズ・イアン]

Hi folks

After coming back from a 2 week holiday, I discovered that I couldn't get to my desktop PC's Ubuntu desktop. After entering user and password information, the information is accepted but the screen becomes a blank purple, and the mouse disappears.

What I've tried so far:
1. I can boot into CLI by adding a '3' to the appropriate line in grub.
2. I can log in remotely via ssh.
3. I can get a remote desktop GUI.

What hasn't worked:
4. Adding 'nomodeset' to the linux line in grub. If I do this, my screen goes black after logging in. (I can still SSH in and RDP in.). If I press the return key, I see a white X in the middle of the screen. I can get a shell with Ctrl+Alt+F3.
5. I messed about with recovery mode, but it didn't seem to help.

System is fully updated including new kernels (I now have kernels 5.0.0-27, 5.0.0-25, 5.0.0-23).

My system: Dual boot Ubuntu 18.04.3 LTS & Win 7. Ubuntu has ubuntu-drivers-common installed.

I've been using Ubuntu 18.04 LTS for a long time, and this has never happened before.

Thanks for any tips or pointers.