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@Wheelnut
Had this problem on a 1950s Claud Butler. Coca cola didn't work, but a strong solution of caustic soda did the trick. This dissolves the alloy but it won't harm steel (although it will ruin the paint). Had to renew the caustic solution every day and within a week it had dissolved the entire seatpin. Obviously, you have to take great care with caustic and it will only work if you can seal the seat tube at one end or the other. It's a last resort really, but it works if you've got a steel frame. Wouldn't want to try it with an alloy frame though...
If you can get a bike shop to remove the seatpin for around a tenner, that's the best and easiest option.