Took matter up with You First as I have a booking to Australia coming up and the reason to access this booking was that I needed to update details of my new passport. Got a very pleasant lady who tried to change my passport details but was unable to. When the problem first occurred I was given the opportunity to have a code sent to my e-mail address rather than it going to my wife's mobile. This gave me access to my account but when I tried to update my new passport details I was taken to a screen that stated the following:
"you are already logged in with a different account. You will need to log out and log in again to access a different scheme"

She has undertaken to flag this up elsewhere within BA by e-mail and to call me back within 2 days. I gave her screenshots of the page in question. Will keep this thread updated as soon as I hear anything.