Thanks again for your help.

Turns out that despite the future flight credit saying "Use by 31 December 2023", when I tried to book a flight (in December) with it they told me they couldn't do it / it had no value because it was more than one year after the original flight. A credit worth ~$8k gone to dust despite what I had been told by the previous chat agents

They have tried issuing a refund request on my behalf (which I don't want) and it was refused (which is normal given the fare was non-refundable). Is there anything else I could try to do here? Is there someone I could ask to extend the validity of the credit somehow?