I know that this train runs every 30 minutes and it is cheapest to buy the tickets ahead of time but I have what is perhaps a silly question. When you buy a ticket in advance are you committed a particular departure time? I ask because I need to get into London as quickly as possible after we land but don't know how soon I can get to the train based on the possibility of flight delays or massive Customs hold ups. Does the ticket you buy cover a window of departure time or do you have to commit to one departure time. I hope that makes sense? Thank you very much!