Headshot of a juvenile Southern Black-backed Gull, locally also known as Kelp Gull.
Well that is what I decided after consulting my bird books.
First I thought it was a Hartlaubs Gull but that one only occurs at the coast along KZN and (partly) the Eastern Cape according to ‘The Complete book of Southern African Birds’.
Now it seems that there is another book which shows that this specie only occurs along the coast of the Western Cape.
South African bird books can be very confusing.