I think I see what you are doing now. JNB-LHR-AMS // AMS-LCY // AMS-LHR-JNB
Next time you do this, I would recommend planning your first AMS sector the day following your JNB-LHR sector. This way you can check your bags to LHR and some time the following day do a back-2-back LCY-AMS-LCY. You get up to 24 hours to connect before paying the additional fare and taxes for the stopover.