I live near the Northern Kwa- Zulu Natal coast and this tree is growing in a neighbor's garden.
you might want to try the tropical fruit forum.. or the fruit forum ...
There is a "Name That Plant" forum here on Gardenweb. I've seldom seen the experts there stumped. If you haven't tried posting there, that's where you'll probably have the best luck.
If it's indigenous, you might be able to find it in a Palgrave's "Trees of Southern Africa". Your library should have a copy. Or maybe try contacting Kirstenbosch and see if they can help.
My first guess was a Combretum but the appearance is not quite right. Someone will know.
Oh, Come on guys that is Medlar.