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 provide either yr flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at stansted-airport station but you may be asked to provide them on your travels from stansted-airport station to the airport.
Going to ashford-international airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap tickets from stansted-airport 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 five but under 16 per adult travelling by train to the airport.
be sure to leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from stansted-airport station before you go. If you end up with spare time at ashford-international airport you can always read a book or play travel scrabble again! As you have bought yr ticket in advance, you don't need to worry about the opening times of the station.
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