Mix the flours, salt, yeast, sunflower, caraway, and poppy seeds in a large bowl. Add the black treacle and water, then mix well. If the dough seems a little stiff, add 1 tbsp or more extra water. Mix well, then put on a lightly floured work surface and gently knead the dough for 7 mins. Put it back into a lightly oiled bowl for approx 2 hours until doubled in size.
Line a tray with baking parchment. Tip the dough onto a lightly floured work surface and knock back, then gently mould the dough into a ball. Roll the dough in the sesame seeds and place on the baking tray to prove for a further hour until doubled in size.
Heat oven to 220C/fan 200C/gas 7. Cut the top into criss-cross slashes with a sharp knife and bake for 30 mins until golden brown and the loaf sounds hollow when tapped underneath. Cool on a wire rack.