To take advantage of the best prices you will need to prove you are a leisure traveller, en route to stansted 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 the station but you may be asked to provide them on your journey from the station to the airport.
Going to the airport on the 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 tickets from greenwich station to stansted airport is that there must be at least two adults travelling together. But another great benefit is that children go free. All children under 5 go free, and then one child over 5 but under 16 per grown up travelling on the train to stansted airport.
be sure to leave enough time to get to the airport early enough to eat, shop and so on, so check the times of trains from greenwich station before you go. If you have time to kill at stansted 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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