Hi,
It's WORKING 
I now understand a little bit more thanks to the kind help of you good folk and the logger is recording our power usage as I type.
One of the problems I was having turned out to be the power supply - I was running the Stalker off the Seeedstudio Solar Sheild with LiPo battery - this casued the sensor to flicker on and off very quickly. Swapping to a 9V battery straight in to the Stalker has solved this, I guess it must be noise of some kind? Not sure if I can solve this - the solar shield is very neat - ideal for a data logging application.
Cheers.