Oh, gooodness Lewis; you have taken on a project!
Been there myself, by way of helping neighbors.
I can tell you what has worked for me...
Resolve to yourself: This seatpost will not survive. A large bench vise or pipe wrench will ultimately be involved in all operations. Or a saddle (as clamp-on lever), which will become disposable as well. So will time. Lots of it. Do not expect results in less than three days' trying if the 'post is really affixed.
1)
Remove the seatpost bolt. Loosening it is not enough 'cos the bolt still retains the "ears" in alignment and they need to be free to spread, no matter how little. Despite what most people actually do, I would heartily recommend
not prying the seatpost lug "ears" apart. It is very easy to distort the top of the seat tube and it can initiate cracks at the base of the kerf/slot below the bolt. Besides, the source of the galvanic corrosion is nearly always lower in the frame.
2) Remove the bottom bracket, invert the frame, and squirt penetrating oil (up) the frame to the bottom of the seatpost as I do the same (down) from above. My favorite is a polymer-based solution called "PB Blaster". Kroil is very, very good as well. For more along this line, see:
http://forum.saxontheweb.net/showthread.php?140464-Penetrating-oil-comparison3) Household ammonia, same routine as the penetrating oil above.
4) Dry ice on the alu seatpost (watch your hands; it burns in a blink).
5) You can try heating the 'post. It won't remove it (it will expand slightly, if anything), but this can be enough to break the galvanic bond during expansion. Wait till it is thoroughly cool before trying again to remove it.
6) There's always the frameless hacksaw. This is brutal on the operator and, occasionally, the frame (unless one is very careful and the seatpost doesn't go in very far).
These are the methods that have worked for me in actual use. For the definitive guide, see Sheldon's recommendations here:
http://sheldonbrown.com/stuck-seatposts.htmlBest wishes and good luck,
Dan. (War, plain and simple...)