Barry,
the vent system is definitely mechanical and fixable with a bit of patience and some triple jointed dexterity(but what isnt). The face level vent assembly can be removed after prying out the individual vent units. Small screwdriver betwen the outer frame and each vent at one end will pop these out, watch out for the little leaf spring tensioning clip though, they like to drop down onto the radio! The main vent unit is held by a couple of screws behind the knobs and a pair of spring clips on either side toward the top. This should reveal the face level flap mechanism. Make sure the foam seal is intact.
The main vent flap controls are prone to sticking in the cable run or at the heater tap itself. A look at how the cables run explains why. You should be able to reach up on the drivers side of the heater box and try moving the lever where the cable hooks up. If its very stiff there is a good chance the control knob end has given up the ghost becase of this stiffness. There have been some threads in the past describing inventive ways to overcome this problem but as you say the heater control works then this is probably OK
From the passenger side the main floor / screen flap control is accessible with the glovebox removed, same again, see if the cable end lever will move by hand to redirect flow. Fairly uncommon for the actual heater box internals to malfunction, normally its the cables or control knob system.
Ian