The gap was troubling me as I had managed to get the Bowden tube seated fairly solidly in the throat of the heatbreak. I had managed to extrude filament, let the hotend get cold, heat up and extrude again a number of times and then the jam again. Turned out that the Bowden tube was moving in the connector, sometimes there was a gap, sometimes there wasn't. I've added a clip to the connector and it seems to have fixed the movement. The top of the heatbreak should not get hot enough to melt filament, but it seems that I had some heat creep. I have removed the all-metal heatbreak and replaced it with one with a PTFE tube, which seems to have solved that. I've managed to get it printing again, but with some bad stringing, I will recalibrate. I've had no more smoke or burnt filament, but I will replace the thermistor when I install the cable chain.