To take advantage of these prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to provide either your flight details or your holiday booking reference. I don't think you will need to provide this when you arrive at clacton-on-sea station but you may be asked to provide them on yr travels from clacton-on-sea station to the airport.
Going to heathrow airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these inexpensive 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 kidover five but under sixteen per adult travelling on the train to heathrow airport.
make sure you leave enough time to get to heathrow 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 a game 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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