Preheat the oven to 200°C/400°F. Lightly oil a 600g non-stick loaf tin. Sift the flours into a large bowl, then stir in the bicarbonate of soda and salt. Make a well in the centre and add the buttermilk, melted butter, sunflower seeds (if using) and golden syrup. Using a large spoon, mix gently and quickly until you have achieved a nice dropping consistency. Add a little bit more buttermilk if necessary, until the dough binds together without being sloppy.
Put the mixture into the prepared loaf tin and sprinkle over the porridge oats with the pumpkin and sesame seeds, if using. Bake for 40 minutes, until cooked through and the loaf has a slightly cracked, crusty top, checking halfway through the cooking time to make sure that it isn’t browning too much. If it is, reduce the temperature or move the loaf down in the oven.
To check that the loaf is properly cooked, tip out of the tin and tap the base – it should sound hollow. If it doesn’t, return it to the oven for another 5 minutes. Tip out onto a wire rack and leave to cool completely. Use the brown soda bread on the day it is made and cut into slices to serve.