To take advantage of these prices you will need to prove you are a leisure traveller, en route to heathrow airport, so you will need to provide either yr flight numbers or yr 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 give them on yr journey from clacton-on-sea station to heathrow airport.
Going to heathrow airport by 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 tickets from the station to the airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one child over five but under sixteen per adult travelling by train to the airport.
be sure to 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 have time to kill at heathrow airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to worry about the opening times of clacton-on-sea station.
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