It seems like the pedants of pprune have shown that this oldphart has confused some 40 year old memories.
Perhaps a few senior moments are not a great surprise at 3 score and 13. It is entirely possible that I have conflated some of many JNB-LHR flights into a single "memory". Some of these flights were BA, some SAA and later KLM to Bristol via Schipol to avoid the mess that is LHR, ending up very close to home.