If I had ample connection time in JNB (which it appears you do), I would personally prefer to pick up my bags and re-check them in for the next flight.

I have heard no end of "luggage missing the connecting flight" stories irrespective of short or long transit time from fellow travellers/tourist using JNB. If you are head for a safari camp, then remember it is not easy to pick up extra clothes or toiletteries if you are in the middle of the bush!

You are travelling through South Africa (international to international connection) you do not have to clear SA immigration which would require a SA visa put in your passport but will have to clear security as with any other international airport. JNB has one international terminal and one domestic terminal so all international flights will be from the same airport.

If your ticket is LHR-JNB-LVI, as oppose to two tickets the the maximum baggage allowance applies.