Quick update:
I did submit an online complaint on the Expedia website but didn't receive any acknowledgment or reply. Eventually I did post a letter (an actual, paper letter) to Expedia Customer Relations and a week later, got the remaining £460 refunded through my Amex card. The booking I opted to cancel in late August has now been fully refunded.

That said, it took nearly 4 months to get that final refund and I still think the whole process could have been, should have been more straightforward.



For reference, here is the address I used to contact Expedia:
Expedia Customer Relations
PO Box 70720
London
EC1P 1GW