Seems to be sorted now! I reviewed /boot/config-user.txt and saw an entry for dtoverlay=gpio-ir,gpio_pin=17
I am not sure where that came from, because my IR receiver is connected up on pin 18. (I guess pin 17 is the default value, and that got used rather than carryover from actual user settings?)
Anyways, I modified the line to dtoverlay=gpio-ir,gpio_pin=18 and rebooted. Now the remote works again!