Heathrow Express has a number of disruptions due to maintenance work up til the end of the year. Be sure to check on their website and block out those dates from purchase.
14/09 & 21/09 - No Service after 23.12
15/09 & 22/09 - First service at 08.25
19/10 - No Service
09/11 - Amended service after 20.30
10/11 - Amended service before 6.30
24 to 27 December - No Service
Amended services just means that there is a train every 30 mins instead of every 15 mins.