Thank you everyone.
I've taken everything on-board and I'm working on it now. Like I said, I'm a newb, so it'll take me a while to get my head around it, but I'll post back asap with an update.
With regard to the question about why I'm using an analog pin, this project ( http://itp.nyu.edu/physcomp/sensors/Reports/PIRMotionSensor ) was my starting point, and I built my code from there. It is this exact PIR that I am using.
Thanks again.