I now need my ticket reissued and since I am back in CPT I e-mailed the request to the CX fares office in JNB. This is the verbatim reply:
"Pax can go to any CX office wherever they are and get ticket re-issued as ONE WORLD rules are the same worldwide". Oh, that really helps. So I phone CX and explain that I am not going to travel to JNB for a reissue. The very helpful lady agrees and says send an e-mail with the request. I say I've just done that! She says she will speak to the fares person and revert. Fares person phones me. It takes a while to persuade her that I'm not in London (the last segment used ended there) but have used another means to get to CPT. Once she fathoms that she says I must go back to the TA that handled the original issue. Oh brother, I really was hoping to avoid that!

TA sends the requested changes through for pricing and CX responds that I can't use EI as it's no longer part of OW! I have provided my TA with the OW information and the ticket conditions to challenge that, but it looks like this is going to be just as arduous a process as the original issue was. And all because of the fact that the only person at CX in JNB who deals this is an ignorant PITA.