I took BA 55 last month. We were about 40 min late in departure but arrived on-time. As others indicated, there is now a premium immigration queue at JNB which I used. However, we were the only flight arriving at that time (most of the other European arrivals come in an hour or so later through the morning) so the immigration hall was essentially empty. We also had a separate ticket departure to CPT about 2.5 hours later on Airllink and we had more than enough time.