Preheat your oven to 180°C (350°F).
In a large mixing bowl, combine the self-raising flour, whole-wheat flour, All-Bran flakes, oats, raisins, seeds, sugar, salt, and baking powder. Whisk together until evenly mixed.
In another bowl, combine the melted butter, sunflower oil, buttermilk, and lightly beaten eggs. Whisk until well blended.
Pour the wet mixture into the dry ingredients and stir gently using a spatula or wooden spoon. Mix until just combined; be careful not to overmix.
Transfer the mixture to your prepared baking tray. Spread it out evenly and smooth the top with a spatula. Bake in the preheated oven for about 30-35 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Once baked, allow the mixture to cool in the tray for about 10 minutes. Then, remove it from the tray and place it on a cooling rack. Once completely cooled, slice it into even pieces.
Arrange the sliced rusks back on the baking tray in a single layer. Return them to the oven and bake for an additional 15-20 minutes until they are crispy and golden.
Let the rusks cool on a rack before storing them in an airtight container.