Thanks folks. I've already chopped all the wood off and I started off
angle grinding the stumps of the bolts off, though decided no to do
them all given the problems I was having. The nuts haven't been a
problem as they either undo or snap off. It's the shaft which has
welded itself into the cast iron.

I think the long term approach is to leave them a few weeks immersed
in oil, then some heat and gentle hammering. Who knows some of them
may then turn with mole grips.

It's the ones that I cut off flush intending to hammer out with a
drift that are likely to be the toughest I guess.

I just looked up electrolytic derusting. That could ultimately be the
answer though may take a while.