Woody wrote:could be wrong but I also think there a little white button on the cadence sensor which puts it into 'discovery mode'
There's no discovery mode on ANT+ sensors. It's only the reciever (Garmin watch, Trainer road software and so on) that you need to tell to look for new sensors.
The sensor units, whether Speed, cadence, power, HR just start broadcasting their output to any device that wants to listen.
That little white button on some variants of GSC10 (and others) is some kind of reset button. After you press it you'll see a coloured LED flash each time the unit is triggered for the 1st 100 triggers or so. Speed & cadence give a green or red flash - can't remember which way around it is - just so you know it is working.
The ones without the button will flash the light when they are first used after not being used for a long time. Again, it'll stop after a minute or two to save power. Not sure how long you have to leave them to make the light thing happen, but it is longer than you normally stop for on a ride so it doesn't keep triggering and flatten the battery.
Useful little feature to see if the unit is working.
