Google to the rescue again. In case this helps anyone else avoid locksmith callout charges on a Saturday, I started off here and here, and was sufficiently enlightened as to solve it myself. Try checking that the key is in the correct orientation to be removed, twisting using a screwdriver where necessary. Then you should hopefully be able to use another key to push it out from the other side, and use tweezers to coax it out the rest of the way. The suggested superglue method wasn't necessary, but is a nice trick.