I'm not a chemical engineer after all

Patience. Research. Planning. Determination. They all played a part. As did my fear of messing about with caustic soda.

A dig through the scrap pile up-cycling, artisanal material section, plenty of grinding and here we have it. Crap photo, but how many phone calls have you made on your Nikon D4?

On the left is the bottom end of the seatpost, on the right is the protruding bit of the tool. Just enough of a lip to hook the seatpost. With some serious "pull hammer" action it came out, not without a fight, but gracefully resigned to it's fate. Park Tools take note ;)

Done. Out. Sanded. Polished. The post and it's saddle clamp re-epoxied.

Given the state of the inside of the seat-tube and the fact that I don't need 300mm of seat tube in a farm gate frame like this, it will be shortened.

Thanks to Graeme for the technical advice and to Babble for querying my commitment to the chemical option