In oups.com,
CJB typed:

... we saw numerous displays clearly stating that
Travelcards were NOT valid on the Heathrow Express.

I travelled on Heathrow Express with a Z1-6 ODTC on Sunday as I have
done several times when the facility has been available. The only signs
that I saw said that Travelcards other than Z1-6 were not valid.

Are you sure you are not mistaken? Did you take a photo of one of these
notices so that you could send it to TfL to complain that Heathrow
Express were not acting as contracted to do.


--
Bob