Jimster:
I'm just using an atmega 328 on a breadboard, there is nothing connected to pin21 Aref, is this where I've gone wrong?
No, nothing should wire to the pin unless you are using a analogReference(EXTERNAL) command in your sketch. Some recommend wiring a .1 ufd cap from Aref pin to ground for added noise filtering.
Check all your wiring between sensor and arduino board. A link to your sensor might be useful.
Also check your breadboard wiring and be sure you are using proper bypass capacitors on the AVR Vcc and Avcc pins.
Lefty