I have clumps of osteospermum in my garden which have been there for years, all grown from one original purple plant, whose name I do not know. I live in a milder part of UK though. I defer to Verdun in all things horticultural, but just have the comment that I find white ones are less hardy than purple ones and fancy varieties with twisted petals less hardy than the older plain daisy-like ones. Verdun, Cannington Roy is white, isn't it, so maybe I should give it a go. I cut mature plants back hard in early spring to prevent woodiness and they grow and flourish again. Cuttings of my purple osteo root very easily (even if just stuck in the ground) but they like to be fairly dry and have free-draining soil and sun.
All this is no answer to the question but I'd be interested to know what others have found.