Increase the air pressure in the decanter by warming it in hot water - probably best not to use boiling - leave it stand for ten minutes to warm up the decanter. Have a cup of tea. More hot water around decanter and wait a bit longer so the air inside is really warm. Wrap in tea-towel and grasp firmly - have strong friend do same with the stopper. Twist and pull at same time. Bag of ice around stopper might help it contract while you warm the flask but not sure.
Richard Buxton, October 2008