Thank you for the link. I note that another member has been having the same problem, and your suggestion doesn't appear to have worked for them. Losing the £1 for a week is a little annoying, so if they work by telephone then I will use that method for the time being. It is unclear if the problem lies with BA or 3V.

BA have confirmed that they had to manually adjust the booking to apply the deposit which wasn't taken correctly before, and this has caused MMB to show figures different to what I had expected. The advice was, as long as the remaining balance is as expected, then all should be OK.