I flew this route last month and had the opposite results.
LHR-JNB in First with a fantastic young female Italian CC serving me and my friends in 1-4A. The service from start to finish was possibly the best i've experianced on BA (I am very critical of BA at times).
I too had pre-ordered the lamb shank and wasn't looking forward to it but it turned out to be very good (bone still on the dish).
However on the return JNB-LHR in CW it was the opposite (although I felt sorry for the crew who had arrived in JNB that same morning and went to the hotel for the minimum time allowed to sleep to return the same night). I slept most of the flight but my friends said the service from start to finish was awful with drink top ups impossible to get, main courses forgetting to be delivered and being ignored during the breakfast service. I did really enjoy the new white company bedding and slept for nearly 9 hours.
For me the crew is everything on BA flights as the hard product is mediocre at best. When you get great crew in F its a lovely way to fly. However, when you get inexperienced or tired grumpy crew it makes you question why you booked it in the first place.