According to the Doctor Who manual the bi-directional motor control board is A-15680. You can get one here:
However, pindorabox has the A-15782 motor controller board listed for the Doctor Who machine:
http://www.pindorabox.com/game-specific/dr-who/motor-controller-board-a-15782.html
Why the difference? I'm assuming the A-15782 is not a correct listing on pindorabox?