In a large bowl, combine bread flour, wholemeal flour, polenta and salt. Crumble the yeast into a separate bowl and mix with the water and warmed milk. Make a well in the centre of the flour mixture and pour in the yeast mixture. Mix until the dough begins to pull together.
When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, 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. Flatten the dough and gently knead in the pumpkin and sesame seeds. Form the dough into a single round loaf and place on a lightly oiled baking tray. Cover with a damp cloth and let rise until doubled in volume, about 40 minutes. Meanwhile, preheat oven to 200 degrees C.
Brush loaf with remaining milk and sprinkle on the sunflower seeds. Bake in preheated oven for 30 to 35 minutes, or until the top is golden brown and the bottom of the loaf sounds hollow when tapped. Let cool on a wire rack before serving.