Thanks for the help so far.
Well. I've created /ess folders in /lib/firmware, and also in /usr/local/lib/firmware, and copied the file to both locations, with no change. It's still not finding it.
I take the point that this would not prevent the system from booting.
As far as dmesg is concerned, I'm a bit stuck with that. The boot process hangs just after apparently successfully starting 'GNOME Display Manager', as shown in
this view. (Can't get the picture clearer, I'm afraid). I don't know how to get to display dmesg when it's hanging like this.
I can, of course, reboot in recovery mode and then access dmesg, but surely that would be showing the trail of that reboot into Recovery Mode, and not the normal boot process? Shall I access that anyway?