Boil milk and mix in the butter and sugar and set aside to cool.
Dissolve the yeast in the water and set aside for 10 minutes till the yeast foams. Meanwhile, sift the salt and flour together in another bowl.
Combine the milk with the yeast mixture; then mix in the sweet potato to form a thick but smooth batter. Then add the oats followed by the flour. Use a wooden spoon and then your hands. The mixture will be fairly sticky, so transfer to a floured surface, knead to make a smooth dough. Put it in a large, oiled bowl, cover with a plastic bag (to allow for increased volume) and leave to rise in a warm place for 3 hours.
Divide the dough into 2 halves, shape into rounds and place on a large, greased baking sheet. Cover with a tea towel and leave it to rise for a further hour till doubled in size.
Preheat the oven to 200 degrees C. Score the loaves, brush lightly with milk and sprinkle with some oats. Bake the loaves for 40-50 minutes till bottoms sound hollow when tapped. Cool on racks before storing.