Stapelia grandiflora is a flowering plant in the Stapelia genus. It is commonly referred to as the Carrion Plant, Starfish Flower, or Starfish Cactus, although it is not related to cacti at all. The name "Carrion Plant" (due to the odor emitted by the flowers as a technique of attracting flies in areas where other pollinating insects are scarce) can also refer to similar Stapelia species as well as members of related genera, including Stapelia gigantea and Orbea variegata. Stapelia grandiflora sometimes also goes by the name of Stapelia flavirostris.
So, not a cactus at all. I take it that it smells of rotting meat but I didn't get close enough to sniff it (to be honest, if I had I wouldn't have been able to get up again).
This garden continues to delight and surprise us.