JNB is a tricky one with acclimatisation as its only +2 hours from the UK. On average it takes 1 day to acclimatise to a new time zone of within +2 hour according to EASA therefore they will technically become acclimatised to JNB time. Saying that the crew will have operated a long duty down to JNB, even more so if they have operated to DUR then fly to JNB, as I think they have a 3 hour wait at DUR for the flight down to JNB, then the drive to the hotel, its like 18/19 hour duty time from memory. They also only get 1 night in JNB. The maximum duty period starting between 06:00-13:29 is 13 hours when acclimatised so even if they are acclimatised in-flight rest is needed.
Its the first season of this new JNB flight however BA will listen to the feedback of the customers using the service and maybe tweak it. Also it might be surprising to hear but our service standards do change on a monthly basis with slight updates!