Just realised that my seatpost no longer needs a bolt to hold it into the frame.
Tried a fair bit of force but no joy.
Took the botton bracket out and sprayed a bit of chain lube down the seat tube which made no difference.
Rather than putting it in the vice and risking bending the frame and/or snapping the seatpost, I'm up for any suggestions.
There's nothing actually wrong with the seatpost height now, so if there are no easy tricks I think I'll just leave it.