Travelling from London, Heathrow to Lusaka, via Jo'burg, how much time should we allow for the international connection in Jo'burg? Is it a matter of arriving at one gate and leaving at another, or do you have to go through passport control, customs, security?
From what I can remember of traveling through OR Tambo, when you arrive you will have the option to go directly to your next departure gate (as long as your baggage is checked through already). I did some reading of other posts on this site which were tagged for Johannesburg and it seems my memory of the process is correct.
The airport is not as big as some of the EU airports I have been in, so you likely won't need much transit time.
I'll add my anecdotal evidence to Paul's - we passed through on an international transit from Zambia to Australia a few years back and it was a fairly straightforward experience. Luggage checked right through as both parts were with SA Airways, and less than an hour to go from gate to gate. I'd allow at least 1 1/2 to two hours just to play it safe though, because some of the gates requiring busing out on the tarmac which can be quite time-consuming and they expect you there some time in advance.
If you do need to collect your luggage for any reason though, the estimate our travel agent gave was three hours and I agree with her. Collection of luggage, progress through customs, re-checking in and re-passing through security is very time-consuming and we had to do this more often than not on our African travels (Sydney-Johannesburg (overnight)-Nairobi, Nairobi-Johannesburg-Cape Town, Cape Town-Johannesburg-Zimbabwe).