Bit of a puzzler, this. If there is more than 1.5mm of friction material on the shoes, I suspect that either the cable itself is incorrectly fitted or hopelessly bodged, completely out of adjustment or (and as mog says, this is by far the biggest cause of Cinq/Sei handbrake problems) that the actuating arm "elbows" are seized. Come to think of it, those could be incorrectly fitted or damaged.
Adjustment of the handbrake on these cars is very simple. There's a threaded screw and locknut -- just follow the cable up from underneath the car and you'll find it (often obscured by a heatshield which runs under the car). just adjust until 2 clicks lock the wheels (but the wheels must be free to turn with the brake off).
All but the nitty gritty is well covered in the Haynes manual, IIRC, the dirty stuff has been covered here many times.