If you want to get into CPT your best bet would be to book the LH fare and book into the 4 digit LH flightnumber which will be the SAA flight (best seats). This way you will get the LH price and the SA seat. Don't take the LH metal flight, the old seats are a pain on the 12 hour flight.

Alternatively you might want to look at Qatar Airways. They have got a thrice weekly flight DOH-CPT and at least here there pricing is very competitive. Less than the equivalent of Sterling 1.000,-- in C (books in D).