I should now bring some closure to this thread, and indeed a thank you to Refund Helper. The issue was not solely related to British Airways it seems, but my new Chase UK account. Full details below for those interested...

When you make a transaction with BA via. debit card, it authorises the full amount as you would expect (for me £146.14). Two days later it will split the transaction into two, assigning each half the value and a ticket number in the reference section. As the booking failed to ticket (for reasons the supervisor had never seen before), the system gave up.

Fast forward to May, and a new 'request' for one halve of the charge was made and to alter the authorisation. As so long had passed, only one charge made it (£73.07) with a ticket number but the second failed as the authorisation was from so long ago.

Someone in ticketing clearly realised it wasn't complete, so, two weeks later went and pushed through the two charges again, this time they both went through (and explains the new ticket numbers).

BA confirmed they have not actually received the earlier funds, and Chase Morgan confirmed 'the retailer has not claimed these funds, so we will redeposit them within 24 hours'. A tad frustrating with BA for just blindly charging my card (but I understand why it did), but also with Chase for not automatically refunding the money which I know both Monzo and Amex do do. But all is sorted!