Just received an email from Heathrow Express and thought I'd post the contents here (my apologies if this has already been discussed: I'm not really following the England forum much this year (going to Italy!)):
You may be aware that changes are happening in stations at Heathrow including the installation of ticket barriers.
From 20 May TFL Rail, a new stopping service, will operate from London Paddington to Heathrow Terminals 2,3 and 4, replacing the Heathrow Connect service.
In preparation for this, we are making changes to some of our terms and conditions to bring us in line with other national rail services and to make sure our tickets work with the new ticket barriers at Heathrow.
From 2 May 2018, the following changes will apply:
Tickets must be used for the direction they were booked. For example, a ticket from Paddington to Heathrow cannot be used for travel from Heathrow to Paddington.
Single Ticket Validity
All single tickets are only valid on the date of travel specified on the ticket when purchased.
Return Ticket Validity
• Outbound journey: Must be made within 5 days of the date of travel specified on the ticket when purchased.
• Return journey: Must be made within 30 days of the date of travel specified on the ticket when purchased.
Peak Travel Times
Heathrow Express Peak hours will be:
• Morning Peak 0630 ‑ 0930
• Evening Peak 1600 ‑ 1900
Ticket Barriers at Heathrow stations
From 20 May ticket barriers will be in operation at Heathrow stations. Heathrow Express customers will still require a mobile or paper ticket to pass through the ticket barriers. Tickets can be purchased online, via our app, at a ticket machine or from our sales team.