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@Russell160
This may be of interest:
http://forum.ctc.org.uk/viewtopic.php?f=25&t=16716
The basic problem is aluminium oxide. Ammonia apparently dissolves aluminium oxide and is apparently recommended by Sheldon. Some also suggest coca cola. Caustic soda just dissolves the seatpost (in theory) rather than attacking the aluminium oxide.
Re. heat, not sure of the theory of this but if you are doing it just apply say, a hot kettle's worth of water. Don't bother applying cold as well. Some say the differential expansion thing doesn't work in this instance because the two parts are chemically fused.