Thank you very much for your swift response!
I had to resort to using gdm. I did run some more updates but they don’t seem to have helped. I will continue to check lightdm’s functionality after updates. Is there any further advice you have in the mean time?
I’ve just run an update for the first time in 2 weeks. On booting afterwards, I’m greeted with this. The second picture shows the result of pressing the menu button in the top right.
I can go to the command line by Ctrl-Alt-F5. There I can log in. Then I tried to run startx, and I get the following result (image at this link - I couldn’t put it in directly because the file’s too big):
I’m totally at a loss for what to do now. I could try downgrading packages, but there are around 150 that have just been updated so I haven’t got a clue which one I should downgrade. Thanks for any help.