I've soaked her in Plus-Gas for two days.
I've been able to break the bonding and gain 3mm of vertical movement; the frame rotated around the seat post, but vertical movement was near on impossible.
I've heated the frame (notice the discoloring), which made things slightly easier in that I got a few more mm of vertical movement but no more.
I've then cut the post leaving 1.5" exposed above the level of the shim. I've clamped the remainder and popped roughly .5 - 1" of the interior post's vertical length away from the wall of the seat tube.
All of this for negligible gain.
Very, very, VERY frustrating.
In the end, I've cut the post at the shim and will be researching local engineering companies who might be able to use a lathe to mill it out. Failing that, a 24mm seat post to go in the hole that remains.
As it stands, I effectively have two shims (and a frame feeling very sorry for itself
Ps - It damn nearly brought a tear to my eye, taking that blow torch to the frame...