well, I was looking at the PCB and noticed through holes labelled cds. I have done some reverse engineering on the PCB, then I found a datasheet which confirms that the controller has a output disable function if the voltage on a pin is <2v http://www.ladyada.net/media/sensors/BISS0001.pdf
I had a 10k LDR lying around and tried it out, voltage was too low because the divider was only being pulled up to 3V via 1 1M resistor. I took it off and connected a 100K pot which seems to give a nice range of light to dark. Very chuffed with these little PIR's and they are so cheap!
Hopefully the only issue I have now is finding a bigger dome fresnel lense