I have an update with some progress!
Through some semi-random clicking* I have managed to get to the Settings menu and to the Key bindings...and my suspicion above was correct - someone's changed the Chat function to LMB! Trouble is I can't edit the binding (LMB just opens the Chat) and what I assume is the 'reset' button (circle arrow button above and right of the bindings) has the same problem, though there is a sound like a button being pressed and the button graphically looks depressed for a moment, however nothing is reset...
So the good news is that it's not a bug and your programming guys are off the hook !
Now we're pretty sure what the problem is (little fingers and tight lips), how can I either reset the controls within the UI, or delete/edit them within File Explorer? As I mentioned, I have uninstalled the game and then manually deleted the remaining Ylands folder under Steam then reinstalled only to have the same issue, which makes me think the keybindings are stored elsewhere...
To prevent a reoccurrence of an issue of this nature, can I suggest implementing an extra stage of the re-binding code that either prevents multiple functions being assigned to the same key, or clears any previous bindings to the newly assigned key before actually reassigning it? Also, I noticed a post indicating that Shift-clicking the .exe in the Steam folder opens a mini-settings menu that contains keybinding settings for controllers and joysticks; perhaps keyboard and mouse could be added here too (or at least a 'reset bindings' control)?
*PS: Although LMB only brings up the Chat box, middle mouse button (MMB) allows me to select the tabs/menus, but not click buttons...By going to the Social tab with MMB I could LMB on the search box without the Chat box opening, and whilst the search box was active (text cursor flashing) I could use LMB to click the Settings button and open the Settings menu. MMB could then switch the tabs there, and also select the different keybinding categories.