I don't think the immigration or baggage handling experience in JNB is relevant here. I'm not even sure why you would collect luggage in JNB though I am aware that you go through a kind of immigration process as you pass (airside) from arrivals to departures. I believe henkybaby has the details.

(There was a recent report of someone being told by a check-in agent that baggage could not be through checked to HRE - I think it was - but the consensus was that the agent was clearly wrong. Apparently the cabin crew may also announce that luggage must be collected in JNB but again that is wrong and only applies for connections to domestic SAfrican destinations.)

Even if you do want to re-check luggage and wish to go through the procedures you should have plenty of time for that - provided your flight arrives on time. My delays have always been caused by sitting on the tarmac at LHR while some or other problem gets resolved. Once you've lost a few hours that way there is no way to make many connections - even ones that are well in excess of mct (which btw is only 60 minutes for int/int at JNB - I imagine the mct should give a fair idea of how things are supposed to work in most cases).