I've had 2 forced overnights at LHR, both on the 57. Overnight accommodation at LHR and then try again at 18:00 the following day. Very tough for the crew as they do a minimum rest turn in JNB and fly back the following night.

In terms of onward flights, if it's CPT then outside major holidays it's pretty easy to arrange a connection. If St Helena, I'd be leaving plenty of buffer.

Rosebank a far nicer place to stop over than Sandton IME.