Thanks, this head has a K type thermocouple on it so no resistance to measure. I will try and check calibration tomorrow, I have a bare thermocouple thermometer somewhere, should be able to pass that down the head to the tip if I can get the filament out.

the filament was feeding very nicely 100mm for an instruction of 100mm so it was set right but the feed just slips when head is fitted, there is no way it would feed as I couldn't even force it out by hand.

i may have made it worse by overheating then so a clean out is in order I think. I'll warm it up and prick it through with some wire etc.