I used caustic soda (sodium hydroxide).
For something so incredibly dangerous, I was a bit amazed that you can just walk into a garden centre and buy it for tuppence.
Remove the bottom bracket, bung up the BB shell with an old inner tube and then pour in hot concentrated caustic soda.
USE EYE PROTECTION!!!!
Seriously, this stuff is lethal! Rubber gloves also essential unless you are tired of your own skin.
You get a mixture of hydrogen gas, steam and left-over caustic soda frothing out of the tube. Not nice.
A couple of nights of doing this and it came out (well, disintegrated).
(Annoyingly, my previous attempts to remove the tube using brute force had damaged the frame so I'm still working out what to do with it).
EDIT: it's not kind to paint, but not as bad as you might imagine. Probably if you put tape over the paint it would be fine.