Insert a long thin flat ended screwdriver (Health & Safety warning - with an insulated handle) through one of the slots in the plastic casing covering the motor assembly and locate one of the two slotted screws which govern the centrifugal switch.

You can identify these two screws by the fact that they have a spring underneath them to keep them in tension.

Try a quarter turn clockwise each then turn the machine back over and try it on the slowest speed.