Yes, I have realised on re-reading the OWE rules that you can only have 2 additional (paid for) segments outside of the continent of origin.

My original master plan was to buy a AONE5 ex JNB and then hang out in the UK and do a number of side trips to Europe. I now realise that once I get to London and then on to either N. America or Asia you cannot return to Europe as you are allowed only one intercontinental arrival and departure.

Interestingly, it explicitly states this in the Oneworld Explorer rules but not in the Global Explorer rules. However, I expect it would be considered back tracking to re-enter Europe a second time on the way back to South Africa.

If anyone has any further thoughts on this issue I would be most grateful to hear from them.