Ok, I warmed the head slowly towards melt point and pulled most of the filament out, it was indeed jammed solid by swelling in the head support bolt, it took considerable force to remove it.

The head is now clear but the jet is solid, could not even probe it when hot with 0.3mm hard steel wire, totally blocked. As it's likely buggered I am going to drill it through with a 0.3mm drill and try that. It could possibly be a bit of swarf in it?

I can see how the heat spread up the support bolt and melted the filament before it reached the hot-end, but what I cant see is how this head could ever work the way it is built. I have seen the same unit on eBay listed as PLA/ABS suitable but they are all 0.4mm, that seems to be the only difference?

Will need to find a way of thermally isolating the support tube I guess or at least a way of bleeding more heat off of it???

Interesting stuff.