The fixings are two pieces. There's a plastic rectangular part, a bit like a rawlpug, that pushes into the square holes beneath the shelf. Then there are some hex headed self tapper style screws that hold the sub up. I did buy some once so I'll see if I can find the part numbers.
EDIT: they are herehttp://www.rimmerbros.co.uk/Item--i-GRID001637
ADU6552 for the plastic bit (called a locut nut, part 12)
DYP100850 for the screw. (part 13)