a VirtualTourist member from San Francisco
Can I buy tickets for the Heathrow Express at the airport or must I purchase them ahead of time on line? Are the ticket offices open late? My plane lands at 20:30 and I expect it will take a while to get through customs and claim my bags.
The www.heathrowexpress.com site has all the info you need. The last train goes at 23.45, so you 'll have plenty of time as long as your plane isn't delayed past 2300 or so!
You can get your ticket from the ticket machines (which take cards, so you don't need to worry about having GBP cash to hand) though it will cost a little more than online reservation. You can also buy it onboard, though it will cost 2GBP more.
As said before cheaper on-line a bit dearer from the ticket machine and even more expensive if you pay onboard.
All relevant information is on their web site.
Trains every 15 minutes up until around 23.30 different days have differing times.