Sorted. Not sure how the effect occurred, but realised the only significant change I'd made was the filament, from white pla to what was supposed to be coffee pla. I tried extruding into thin air, it was slow and the extruder drive was labouring. I raised the temperature from my usual 205 in steps of 5 degrees, and I didn't get good extrusion until I got to 230. There was no smell, so I guess it is still pla, but maybe a higher molecular weight polymer.