I gave it one last go after trying penetrationg oil and brute force. I filled up the vessel with caustic soda and let it bubble away hoping that it would eat through some of the alu shim to get the seatpost moving. I let it work for a couple of hours drained it out and then attached a metal rod to the end of the seatpost clamp to try to get better leverage. I wedged the metal bar between to big static sturdy pipes to act as a clamp (my studio is in an old mill that was run on steam so all the old massive pipework is still in place).
AND...... nothing.
so off to the framebuilders it is.