Paxboy
I think you'll find the departure times ex-JNB are more to do with arrival times at destination rather than anything else. The typical flight time is 10hours to N Europe, so leaving at 1000 gives arrival at 2000 and at 2000 gives 0600, much in between is not verypopular. Yes, JNB is high, sometimes hot, but usually you derate thrust quite a bit - midday 35deg C take-offs would be feasible.