To take advantage 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 your flight numbers or your holiday booking reference. I don't think you will need to give this when you arrive at harpenden station but you may be asked to provide them on your journey from harpenden station to the airport.
Going to the airport by train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap tickets from harpenden station to gatwick 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 grown up travelling on the train to gatwick airport.
be sure to leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from harpenden station before you go. If you end up with spare time at gatwick airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to worry about the opening times of the station.
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