Just to update - haven't got anywhere when trying to deal with this. Whoever I spoke to from the call centre just had no idea understanding the issue - eventually got into some email correspondence with someone at BA.com and received the following six days ago...
Thank you for contacting us again about the difficulties you’re experiencing trying to login on ba.com.
It should be fast and easy to do this online, so I’m very sorry it’s not worked like this for you. It is unusual for this to happen and thankfully the majority of our Members don’t have any problems. I appreciate this is little consolation for the frustration you have experienced.
I'm afraid I haven't been able to fix this for you at the moment, and I've reported the problem to our systems team who have started working on a solution to correct this.
I'll contact you again as soon as we have an update from them, but you're welcome to keep trying in the meantime.
Please accept our apologies for any inconvenience this may cause, and thank you for your patience. If we can help with anything else please get in touch and we will be happy to help.
Why is this such a hard issue to fix?!?!