bmi have unfortunately been unwilling to check through luggage on separate tickets for a few years now - it may be worth throwing yourself on the mercy of the check-in agent when you check in -- or the better option if possible changing to an earlier connecting (domestic?) flight to give yourself enough time at Heathrow.

Its probably worth doing that anyway given that delays are fairly regular and you may run the risk of losing the onward ticket given they are separate.

bmi and South African are both in Terminal 1 at Heathrow so you may not need as much time as you think to change.. but its best to allow more just in case.