Thanks everybody for the useful replies. Everybody is correct: I did not know that /sys is an impermanent logical directory and that /sys/firmware/efi is created ("appears" in arvidjaar's vocabulary) at boot time. Nor did I know that the pmbr_boot flag (that YaST leaves unchanged) might influence the outcome of the boot. In my case, though, it didn't. I removed that flag (using YaST, confirmed with parted -l) before I shutdown the machine yesterday but when I booted the machine this morning it still seems to have booted in legacy mode (definitely no /sys/firmware/efi created/appeared).

karlmistelberger's suggestion to use a 'live' usb disk to see if the machine really can boot in UEFI mode is probably the next thing to try, although that might have to wait as I'm under a bit of pressure to get back to my day job.