I received an Amex Offer for 10% off "Heathrow Express Train: London to Heathrow Terminals" which suggests the cashback is only valid on a one-way ticket in the Paddington to Heathrow direction. I read the T&C's (yes actually) which doesn't mention anything specific except it's only for tickets booked through the website or app (which would seem to exclude contactless fare payments). I wonder if buying a return would invalidate the offer...