The bad news, I managed to blow two of the three motor drivers by connecting 9v into a 5v pin. Oops.
The good news, the remaining driver works fine!
Here is a video of it in action:
Here you can see me reversing the direction of the motor by connecting the DIRECTION pin to 5v or 0v.
The step pin is actually disconnected and is therefore floating. However, because the voltage on it is therefore fluctuating, it more or less simulates a quick step pulse train.
I have ordered some replacement drivers and some opto-couplers as I reckon the next step is to connect the stepper motor to the pc and see it in action. The opto-couplers ensure that, should anything go wrong with the motors, no electrical signal can be fed back into the pc, potentially damaging it.