I did manage to fix the shredder - a little victory but was bamboozled for a while by why it wouldn't turn out. Turns out that there are 3 switches that must be wired in series. The switch you press to turn it on is A. Switch B means that the rotor won't run unless there is paper in it. The paper pushes down at B on the black plastic arm that pivots, pushing inwards to close the switch. When that paper has shredded, the arm pops back out and the motor stops.
But there is a third switch that is only activated by putting the shredder into the waste tub so you can't operate the shredder away from that tub.
