Hi 3crownsamigos
Hope you are well. Sorry to hear you are having these issues.
I myself have not had much experience when it comes to PPPOE details in regards to leased lines. I know that for regular ADSL or FTTC/FTTP services that getting your username/password is something we are able to do on our side no problem, so I would have assumed that it would have been the same on the leased line side of things.
I know you have already spoken with us on the phone, but I would try again directly on 0800 100 2440, which should put you through to our BTNet repair team who would be best suited to deal with your query.
If you still have no joy, get back to me and I will see what I can find out for you once I am back in the office.
Regards
Ryan