XBee + TMP36 temperature sensor - returning inaccurate values
I think I've worked it out. The power was coming through a SparkFun USB Xbee Explorer. This was rigged up into a breadboard, and power was provided by USB from a PC.  

I've replaced this with a basic adaptor, a 3.3V voltage regulator and a 5v power supply. This seems to have given a much more reliable signal. I'll do some more digging to see if there is some noise coming into the circuit somehow.