Heathrow Express Train

My memory is foggy but I don't think you buy a reserve seat, just a ticket for the next train. I think there is a discount for purchasing early so just buy if you do not have to specify a certain time. Otherwise buy it when you get there. While most of my planes have been on time I did have a six hour delay and a 12 hour delay.

For the HEX, you do save 1.50 GBP if you buy the ticket online rather than from the kiosk in the airport. When you buy online, you only enter the date of your trip (not the time). Seats are not reserved. On my last trip, I flew into T5 and there were personnel with credit card machines just outside arrivals selling tickets. They were taking swipe cards.

Thank you for the responses. I need to clarify one question in my original post. I am inquiring about the HEX leaving London. Being unfamiliar with Heathrow and given that there isn't a specific departure time required for the ticket, I don't know how much time I should allow to get to Heathrow from Paddington.

Sorry, I got the wrong direction. but you still don't need a reserve ticket Four hours is reasonable. It is quick from Paddington and a few minutes walk to check in. Heathrow can be a zoo but you wouid be OK if you got o the train at Paddington four hours ahead of your flight.

Going both ways, no reserved seats or reserved times. So you can catch any train thats convenient . the train journey is fairly quick, but be prepared for usual long lines at heathrow- check-in and to get through security.
Prudent to arrive at Heathrow at least 3 hours ahead of scheuled departure, and then get through security promptly

If I remember correctly, last time I took the express my flight was leaving at 12:30, and I took the 9:00 train and had plenty of time to navigate security, check in, etc, etc and then have a nice leisurely breakfast at the airport, wander around ,etc..the express trains to and from Paddington run fairly regularly...can't comment on when/where to purchase tickets, or what the cheapest method would be...I stayed near Paddington on my last trip, so was using that station every day...I just picked up my ticket the evening before my departure from the ticket counter at Paddington...I don't think you need to worry about getting on a particular train, I've used the HEX twice and didn't find it crowded at all, given there are much cheaper options to get to Heathrow from London( ie regular Tube train...just takes longer ) and the majority would probably go that route...