I had a problem with an Amex 2 for 1 booking about 3 years ago (LHR - CPT). I had modified the booking and they refunded me some money and I did not realise at the time that they had refunded it all!... However the booking still showed up and I even managed to check in online.

It was only when I got to check-in at T4 that I found out there was a problem, they labelled my bag up and then said "OK where's your ticket", the e-ticket had been deleted!.. I had to spend best part of an hour at the BA ticket desk sorting it out and I was relieved when they finally let me check-in.

My advice would be to call BA. Even if you have a booking that shows up it does not necessarily mean that you have a valid e-ticket.