Hi
Shouldn't be. Look at flysaa. There is usually aflight around 9pm to Joburg then 10 or 11am to E London. Flights back around 4pm to joburg then around 11pm to Heathrow. How about flying to Port Elizabeth, just booking some for a client and they are around 15 hours inc stop in Joburg both ways.
Hope it works out for you.
Cheers
Adrian