Yes, plane armed and sent on Auto flight. No logs at all on SD card.
I've tried with logging at boot on and off but no difference.
Is there a basic way to enable or disable logging that I've missed?
No, it shoul dlog out-of-the-box.
Setting the parameter LOG_DISARMED should cause it to log all of the time.
Setting the parameter LOF_FILE_DSRMROT should close the current log file
each time you disarm ('though if you are not logging while disarmed it
will only be created when you next arm).
So. To your problem.
Is your LOG_BITMASK non-zero?
Is your LOG_BACKEND_TYPE 1 or 3?