To take advantage of the best prices you will need to prove you are not a business traveller, en route to stansted airport, so you will need to provide either yr flight numbers or yr holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on your journey from the station to the airport.
Going to stansted airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just gaze out of the window... The other caveat to buying these inexpensive tickets from the station to stansted airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All kids under 5 go free, and then one child over five but under sixteen per adult travelling by train to the airport.
make sure you leave plenty of 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 have spare time at stansted airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to fret about the opening times of the station.
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