To take benefit of the best prices you will need to prove you are not a business traveller, en route to the 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 brent-cross station but you may be asked to provide them on your journey from brent-cross station to the airport.
Going to the airport by train is great, you can take a nap, read (even at night), play a game, or just gaze out of the window... The other restriction to buying these cheap train tickets from brent-cross 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 five go free, and then one child over 5 but under sixteen per adult travelling by train to stansted airport.
make sure you leave plenty of time to get to the airport early enough to eat, shop etc, 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 worry about the opening hours of the station.
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