Thanks guys!
I think I've solved the mystery. It wasn't a component-related issue, I had it all connected the way you suggested. So I've decided to give SimpleMessageSystem a go instead of Pduino, just to see if it makes a difference...
Now I'm getting correct values! Both the LDR and the potentiometer now transmit values within a full range of 0-1024! 10-bit! Excellent stuff!
The only thing I'm not very happy with is latency (I guess that's due to value sampling rate or something) - if I twist the knob rather quickly, values jump in huge steps. Like 0-100-300-600-1000-1024 (6 steps). Is there a way to smooth value changes? I've implemented a workaround within Pure Data, but it's not very responsive (values change smoothly, but with a noticeable delay, so not ideal for real-time audio applications). Anyway, I guess I shouldn't be complaining. It's only set me back £20 to build this interface. I can certainly live with that...
Thanks for your help! Very much appreciated!