Martin, there are a few ways to try with different severity.
I guess you've tried using penetrating oil and leaving it to soak over a period of time? "They" reckon to make a plasticine funnel around the joint where the seatpost enters the frame and keeping that topped up with easing oil and leaving it for a period of time for maximum penetration.
Someone mentioned pouring coke down the seat tube.....
You could apply heat from a paint stripping gun.
You could clamp the seatpost in a vice and using the frame as a giant lever turn the frame to release the stuck post.
Or chop the top of the post off and saw very carefully in vertical cuts down the length of the seat post (taking care not to saw into the seat tube!) then use a hammer and screwdriver to gently chisel the smaller strips of metal out. This is only a good idea if you're able to cut the whole length of the seatpost. It also takes a long time so be patient.
Hopefully one of these techniques will work for you, if you don't fancy any of the more drastic versions you'll have to take it to a frame builder (Dave Yates is over at Coningsby)
There might be some more ideas coming in on the board as well.
Don't forget to copper slip the new seat post before you insert it into the frame!