Is it actually showing as a refund, or has the transaction just disappeared?

Years ago I worked for Clydesdale Bank. People were often confused because when they paid for something on their card, the authorisation would show on their account as a debit pretty much immediately. That authorisation would generally disappear off their account within a day or two. The transaction only reappeared on their account when the merchant actually called in that authorisation, typically a few days later (of course, the real hassle for people were the rare-ish occasions when both authorisation and transaction were simultaneously showing as a debit on someone's account!).

Might something similar have happened here? Either way, I'd suggest keeping the money aside (physically or mentally), as BA will probably realise if they haven't been paid, and will seek to put it right.