I've succeeded in assembling the BfB extruder and gave it a test run. It is able to heat the HDPE and can extrude the material but at a rate of only about 4mm/min.
I don't know what the appropriate rate is so if anyone can tell me I'd be grateful. The test bed consisted of a 12V 2A supply for the heater and a 6V supply for the G3 feed motor while using a VOM to monitor the resistance on the thermistor. After successfully extruding a small amount of HDPE I gave the machine a rest and tried it again later. I discovered that the thermistor fried out after only a few hours of use, I'm attributing this to unknowingly placing the thermistor too close to the heating wire.
I took a spare thermistor and wrapped it to the outside tube with PTFE and resumed my tests this seems to be acceptable, so far. I'm considering either increasing the diameter of the extrude hole or determining if the feed path has too much resistance.
I'm also at the stage that I think reconfiguring the circuit boards would be beneficial.