I finally managed to speak with an Apple agent, after numerous failed attempts via email and chat, and she successfully credited the money to my gift cards. (Email and Chat support just kept sending boilerplate replies with links to existing pages without even attempting to read my email.)
I now had the money back but unfortunately I didn’t have the original cards to scan the QR codes and add them to my Wallet again as I’d thrown them away. Fortunately I had taken screenshots of the card numbers and PINs before contacting Support but trying to add them manually kept failing. Then I discovered this website that generates the QR codes for you - and it worked so, so smoothly!
https://ephams.com/2017/11/quick-add-apple-store-gift-cards-iphones-wallet/