It looked as if the ferrule and wooden stub had been badly crushed, so that the ferrule had an oval cross-section and the stub had fractured vertically and lost its structural integrity. The stub would have to be replaced.
I was able to push a screwdriver under the base of the ferrule and lift it by a millimetre or so. I could then get a hacksaw in between the base of the ferrule and the top of the handle and saw through the stub until I reached the screwdriver blade. I could then push the stub remnants down to fill the gap, saw again, repeating until the stub was gone. The ferrule could then be squeezed a little bit more oval and eased over the screwdriver blade. After pulling out the blade, not easy because of the corrosion, I had the following components: