A dig through the
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