Look at page 91 of the manual C-18. I think it is third over Lock AE. I have my AFDII focus set on the AE lock button.
Many thanks - I've got it now! Somehow I missed these capabilities when I first read through the manual.
C-18 takes autofocus off the shutter release.
Then C-17 allows you to choose either the front or rear button for autofocus.
(Discovered by trial and error - the manual doesn't really spell it out.)