Passport control chaos-connection missed-no refund

Last week I landed at OR Tambo from Sydney (a One World reward flight using Avios). Due to the utter chaos at passport control there (freakishly long queues, no help from officials when notified of urgent connecting flights to catch) I missed my connecting flight back to Cape Town, also a reward flight. Both business class. I was told I couldn't be put on next flight & would have to buy another one way ticket for approx R2400! Their reasoning - wasn't booked as a single ticket & thus not their problem. A bit disappointing given BOTH flights were booked through BA Executive Club & I couldn't just be put on the 7pm flight (proved to be an empty flight by the way) when the 6pm one was missed due to being stuck at immigration for reasons beyond my control. So the one way domestic flight effectively cost me R3300 + 7000 Aviois approx - rather ridiculous. Neither email correspondence (customer relations) nor phone calls have proved any help at all.