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