Spent my RR Saturday on improving the heater barrel isolation using glasswool. Then lost quite some time on figuring out why the extruder motor would not work anymore. Finally backed out to Gcode_interpreter SVN version #2483. I suspect the preparations for stepper motor extruders to be the culprit so I will leave the code in development a while for what it is.
Anyway, I tried to get the extruder to work and did not succeed. Better isolation of the barrel only slightly helped in that a little more HDPE comes out of the nozzle now. Tweaking the thermistor lookup table (with createTemperatureLookup.py and setting Beta to 6000 :-) to achieve higher temperatures which didn't help much either. Btw, the heater shouldn't get too hot for the PTFE insulator.
I conclude I did not build the heater barrel correctly. So, I mailed BfB for a new heater barrel kit :-( and for that matter, a second extruder for future (two material) use.