I called BAEC within 24 hours to try and change my ticket.
They were happy to do it FOC but unfortunately the only way to have our bookings on one e-ticket number would be to cancel and rebook. As neither of our flights had Avios availability we couldn't risk cancelling incase the seats didn't go back into the Avios bucket.
They did however put a note in the bookings linking the flights but of course not protecting them in case of misconnect.

Thanks all for your help on this ^ fingers crossed we don't misconnect