After waiting a few weeks I got this reply today from CX:

Thank you for your comment.

Cathay Pacific follows IATA general rules in fare application. Since your ticket was reissued after the promotion period, "Passenger Air Tariff" General Rule applies and is published as follows:

"In the event of voluntary change to the originating flight, the fares and charges for the passenger's journey shall be recalculated in accordance with the fares and charges in effect on the date on which the change is made and is reflected on the ticket."

We regret if you were not aware of the above fare rule and should you have further enquiries with regard to your ticket, please kindly contact respective travel agent or our worldwide reservations who shall be able to assist you.

We thank you for bringing your concerns to our attention, and as always, we look forward to being of service to you soon.

They have missed the point completely regarding the Oneworld rules and merely parroted out the IATA regulation line.

To be honest, I didn't expect anything better for the 1st round of correspondence from CX.

I will now try and get them to focus on the specific issue.