So on Friday I updated to the latest kernel, 184.108.40.206 and also the latest NVidia drivers from the Extra repository, 396.24.2. Upon restarting the LightDM service failed to start, so I installed SDDM thinking it was a LightDM problem. That was even worse as at least when it crashed with LightDM I could still use the tty sessions properly. Eventually removed SDDM and went back to LightDM.
On investigating the logs, it X was throwing up a framebuffer error, and didn’t appear to be trying to start the NVidia drivers. I downgraded to 220.127.116.11 and 390.48 and my system booted correctly again allowing me to access the KDE desktop. It seems to me like the kernel modules for NVidia weren’t being built. Has anyone else come across this problem?