Ok, solved the problem.
There's no button you have to press to "switch" it on. It goes into "sleep" mode after a few minutes if the bike isn't moving, but as soon as the magnet on the spoke passes the sensor, it kicks back in and starts registering data.
The problem I had was caused by 2 things.
The magnet was screwed to the spoke offset to the wrong side. Kinda hard to explain, but the mounting thing for the magnet has a little groove in it to allow it to slide onto the spoke, then the screw with the wide flat head screws in and tightens it up securing it.
I had the mounting thing screwed on so it was sitting too far over to the left of the spoke. I removed it, flipped it round the other way and tightened it back up and then spun the wheel, which resulted in a reading, but only after I had adjusted the sensor further so that it was
almost touching the magnet. I had it slightly too far away, it needs to be really, really close.
Now to get used to all the functions on the thing, to be honest, its very a very clever little gadget, but perhaps a bit of overkill as far as functions and readings go, especially for a beginner like me.