Mango cancel flight - struggle to get refund

I booked a flight from OR Tambo for 7 people months in advance to East London. Mango cancelled the flight 5 days before we were to depart. I had to buy replacement tickets at 4 times the original price! since then I have been struggling getting my money refunded or replaced by a voucher. I mean - THEY cancelled, not me? I think the least the airline could do is refund me promptly without having to BEG them!?