Jpax
"Last time I was a passenger on a flight that diverted (a short haul flight) we waited an hour and the same crew flew us on to LHR. "
Yep, short haul might be able to do that, but with JNB - LGW you might be unable to operate onto LHR because of Legal Flight Time limitations and in addition JNB-LGW would probably trigger some Long Haul specific Union rules.