To take advantage of the best prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to give either yr flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to provide them on your journey from london-kings-cross station to the airport.
Going to stansted airport on the train is great, you can sleep, read a book (even at night), play a game, or just gaze out of the window... The other restriction to buying these cheap train tickets from london-kings-cross station to the airport is that there must be at least two grown ups travelling together. But another great advantage is that children go free. All children under 5 go free, and then one child over 5 but under sixteen per adult travelling on the train to the airport.
make sure you leave enough time to get to stansted airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you end up with time to kill at stansted airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to worry about the opening times of the station.
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