To take benefit of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either your flight numbers or yr 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 give them on yr journey from new-cross-gate station to the airport.
Going to gatwick airport by train is great, you can sleep, read (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these cheap tickets from new-cross-gate station to the 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 the airport.
make sure you leave plenty of time to get to the 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 gatwick airport you can always read or play a game again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of new-cross-gate station.
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