A brief update to my post above in case the data is useful for anyone; the call took just 15 minutes, approximately 13 minutes holding followed by about 2 minutes in conversation.
Very efficient customer service representative, not overly friendly - but perfectly polite, and fast. No doubt working extraordinarily hard under immense pressure, so I’m grateful.
No issues whatsoever with my request for a refund. The word ‘voucher’ wasn’t even mentioned.