Ah, I didn't know they made those.

There was a lock nut on the top, but I had undone that before starting this thread. The fork still stayed in place, so it was a wedge stuck in the fork. I got my hammer out to whack it.

I thought i'd be clever and put it on the grass. I dripped some oil into the fork tube, put a chisel in and started whacking it. When I turned the frame over to see if i'd made any progress, i'd knocked soil into the tube! I had to pick all that out with the chisel - rookie mistake.

I managed to knock it out after a few tries. From there it was a simple task of unthreading all the bits.

Long story short: I did it!