Mix together yeast and castor sugar in 1cup lukewarm water in a bowl. Set aside to activate.
Mix together wheat flour, refined flour, salt and rosemary in another bowl. Add activated yeast, olive oil and sweet potatoes and mix till well combined into a dough.
Dust the worktop with some flour and transfer the dough on it. Knead for 10 minutes or till it turns soft and elastic. Place the dough in an oiled bowl, cover with a cling film and set aside in a warm place for 30 minutes.
Line a loaf tin with butter paper.
Knock back the dough. Place the dough in the lined tin and set aside in a warm place for 20 minutes.
Preheat oven to 180° C.
Dust the dough with some flour, place the tin in the preheated oven and bake for 25-30 minutes. Remove from oven, cool and demould.
Slice and serve.