[CharlesJones]

Thanks for the explaination. I really wish there was a write-up somewhere which explained all this stuff - I don't even understand what the ref sensor is for. In fact, I don't understand what both the ref sensor and the speed sensor are for, or why there are the two of them when they are identical sensors!

Yeah, I suspect a faulty cable - maybe it was my disconnecting and reconnecting the sensor cables which wiggled it and got it working again.