To take benefit of these prices you will need to prove you are a leisure traveller, en route to the airport, so you will need to give either yr flight details or your holiday booking reference. I don't think you will need to provide this when you arrive at brent-cross station but you may be asked to provide them on yr journey from brent-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 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 children under 5 go free, and then one child over five but under 16 per adult travelling on the train to the airport.
be sure to 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 the 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 worry about the opening times of the station.
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