Thanks avioniker, but I'm still bemused:
27-51-00-062
"TE Flap Uncommanded Motion (UCM) Detection
The FSEU compares the data from the TE flap skew sensors 1 and 8 to the data from the flap lever position sensors"
From that, it seems that the system only uses two skew sensors - and they're the most outboard ones, too.