I'll certainly try that - was just about to do the nuclear option and reinstall but would rather avoid if possible.
Some investigation of my own has uncovered the culprit (I think).
Looking at journalctl -b, I started getting messages a couple of days ago regarding a directory in /tmp not being writable and XWayland not starting because of it. This is repeated each time I've booted since.
gnome-shell[1381]: Running GNOME Shell (using mutter 43.8) as a Wayland display server
gnome-shell[1381]: Failed to start X Wayland: Directory "/tmp/.X11-unix" is not writable
systemd[1914]: Starting GNOME Shell on Wayland...
systemd[1914]: org.gnome.Shell@wayland.service: Skipped due to 'exec-condition'.
systemd[1914]: Condition check resulted in GNOME Shell on Wayland being skipped.
So, just now I changed the owner on that folder to root, logged out and back in again and found myself in a Wayland session. After reboot however, the problem returned and I am now back in X11.
Looking at that error online took me to a RedHat page which mentioned a bug with systemd - something about it not creating tmpfiles properly before mounting the directories. Obviously not the same issue, but definitely on a similar vein.
Permission showing in /tmp:
-r--r--r-- 1 gdm gdm 11 Jan 13 11:22 .X1024-lock
drwxrwxrwt 2 gdm gdm 4.0K Jan 13 11:22 .X11-unix
total 8.0K
Permissions inside the .X11-unix folder:
drwxrwxrwt 2 gdm gdm 4.0K Jan 13 11:22 .
drwxrwxrwt 17 root root 4.0K Jan 13 11:27 ..
srwxrwxrwx 1 wheeljack wheeljack 0 Jan 13 11:22 X1
srwxrwxr-x 1 gdm gdm 0 Jan 13 11:22 X1024
Can anyone post their 'working' permissions here to see if they differ?
I just tried creating a new user, and exactly the same there - will not log into a Wayland session.