To take benefit of the best prices you will need to prove you are a leisure 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 the station but you may be asked to provide them on your journey from clacton-on-sea station to the airport.
Going to the airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap train tickets from clacton-on-sea 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 5 but under 16 per grown up travelling on the train to heathrow 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 clacton-on-sea station before you go. If you have spare time at heathrow 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 the station.
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