I bolted together the complete extruder assembly.
I finished off the extruder and tested it. The barrel gets hot, the temp reader goes up and the motor starts on request. Result! or so I thought.
After testing, it seems that it wasn't getting hot enough. I measured the temp with a digital kitchen thermometer, and the difference was significant.
After a bit of investigation, I found I had the wrong include file for the thermistor in the arduino code. Switching it and re-loading the arduino gave me a much better reading - the kitchen thermometer (touching the outside of the barrel) now reads within 10 degrees of the thermistor (buried inside the fire cement).
I also insulated the barrel using a section of silicone plate mat. Tesco's had some reduced at 10p each so I picked up a couple. This equalized the temperature between the heater top and bottom.