To take advantage of the best prices you will need to prove you are a leisure traveller, en route to gatwick airport, so you will need to provide either your flight numbers or yr holiday booking reference. I don't think you will need to give this when you arrive at new-cross-gate station but you may be asked to give them on your travels from the station to the airport.
Going to gatwick 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 restriction to buying these cheap tickets from the station to the airport is that there must be at least two grown ups travelling together. But another great benefit is that children go free. All children under 5 go free, and then one child over five but under sixteen per adult travelling on the train to the airport.
be sure to leave enough time to get to gatwick 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 spare time at the 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 hours of new-cross-gate station.
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