Zara, Gav. I have already tried both wires. Only thing I've yet to try though is the 'negative' wire with a pill-up voltage. I'm going to try that tomorrow night.
Mike - how silly do you think I look
? But to be clear, yes, you pick a speed (I tried 40kmh and a few others to be sure), use your gps or the obd2 scanner to measure 40kmh (while your driving), then press the 'speed matched' button, and it is supposed to then read the signal and lock it in, but obviously it fails.