The pic was taken before I'd attempted anything. Looking at the top of the head tube, I think I can see evidence of removal via dremel so whatever the headset was, it was the same top and bottom.
I'd tried various things with the tools available, but with so little of the race to hit or pry, I returned to the dremel and a larger grinding stone.
A steady hand later I'd managed to create a big gap in the crown race but it still would not budge.
I decided to do the same to the opposite side of the crown race, as much as I dared as I didn't want to damage the frame, and finally after getting it quite thin, a bit of gentle prying with a screwdriver and success:
Now that it's out, I'm wondering if someone hadn't actually pushed in some sealed bearings, as what's come out is nothing like any headset I've come across, maybe someone trying to create their own integrated headset?