Lightly oil a baking sheet with spray oil. Mix the flour, sunflower seeds, poppy seeds, bicarbonate of soda, salt and sugar together in a bowl. Make a well in the centre, add the buttermilk and gradually work into the flour mixture to form a soft dough.
Turn the dough out on a lightly floured work surface and knead for 5 minutes. Shape into a flattish round. Transfer to the prepared baking sheet. Using a sharp knife, cut a cross in the top of the bread. Sprinkle a little extra flour over the surface.
Bake in a preheated oven, 230°C (450°F), Gas Mark 8, for 15 minutes, then reduce the temperature to 200°C (400°F), Gas Mark 6, and bake for a further 25–30 minutes until risen and the loaf sounds hollow when tapped underneath. Leave to cool completely on a wire rack.