In a small bowl, dissolve yeast and sugar in warm water. Let stand until creamy, about 10 minutes.
In a large bowl, combine the yeast mixture with the oil, margarine, salt and 200g flour; stir well to combine. Mix in the remaining flour, 50g at a time, beating well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and supple, about 8 minutes. Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 1 hour.
Deflate the dough and turn it out onto a lightly floured surface and form into a loaf. Place the loaf into a lightly greased 450g loaf tin. Cover with a damp cloth and let rise for 15 minutes. Meanwhile, preheat oven to 180 C / Gas 4.
Bake in preheated oven for about 30 minutes or until the top is golden brown and the bottom of the loaf sounds hollow when tapped. Cool in pan 5 minutes and then turn out onto rack and cool completely.