A response from Diamond Bus

”The driver is correct that the primary objective of this service is to provide transportation to Heathrow Airport employees, and supported financially by Heathrow Airport for this purpose.

However, this service is open to the general public, and drivers have all been informed of this, so this situation will not be repeated.

The date on which you travelled was shortly after launch, and during recent weeks, we have successfully recruited a large number of new drivers, to improve our service delivery.”

Still annoying and no offer to reimburse taxi fare, but if any readers are faced with the same situation…