The marriage register entry is available online.
It shows addresses for both of them, his occupation, and names of two witnesses. In his address, the more usual spelling of the suburb is Vrededorp. Vrededorp and Cottesloe are both notable enough to have ther own Wikipedia articles. You might try to find out what sort of institution the Girls' Diocesan Hostel was. It is not quite clear whether Cottesloe was in the Diocese of Auckland Park at the time (not impossible) or whether there was more than one Diocesan Hostel.
The name is unfortunately not highly distinctive. At least one other one, also born in 1920, left an online trace at FamilySearch. Three people of that name have estates on file with NAAIRS in the MHG collection.
No Stephanus Johannes Jansen van Rensburg born 1920 was on the 2004 Voter's Roll.