Hi Johnny Oakes, first you need to speak with billing and ask for the ebilling login details to be changed to any other address - i suggest a non BT email - hotmail etc.
Then once that has been done speak with the broadband team who will be able to set up this address.
I suggest speaking with the livechat team as you will have one long transcript to where billing agents and broadband agents can referback to see what the other has done.
The link for these can be founde here - contact us
I hope this has been helpful