Which issuing authority's Class 1? I don't think that the SA CAA would recognize anyone else's Class 1 Medical. They'd want you to pass theirs. While the British Class 1 only requires a resting ECG the South African Class 1 initial and certain age related renewals require a stress ECG.
By the way it is not actually the SA CAA that undertake the medical recognitions or authorisations. SA Class 1 medical applications and reports still go to MMI (Military Medical Institute) at Irene and it is in effect the military who have the final say over your SA civilian Class 1.