Ok, update for you, me, er... (ever get the feeling you're talking to yourself?)
I have used another multimeter (high quality) to measure the impedance of the ADC-16. It worked! (after a fashion).
The problem I have now is that the multimeter outputs 1.01V (as can be seen in the PicoLog software) to make the measurement, this means that I have measured the impedance of the logger's 1V range, does the ADC-16 have multiple ranges? Is it autoranging? (I would imagine so)
If this is the case I can "force" the ADC-16 into the appropriate range by placing a known (very) large resistor in series with the ADC-16 but this makes the measurement a little more... unstable, and it's hard to get an accurate reading.
I hope this simplifies the question, basically I need to know if the ADC-16 has multiple range, and if so, what are they?