Heat oven to 180C degrees. Grease and line a medium loaf tin.
In a large mixing bowl, mash the bananas with a fork. Add the oil and sugar, then gently combine.
Add the flour, baking powder and spices, then combine well.
Add the walnuts and sultanas (if using).
Tip the batter into the prepared tin, spreading it out evenly.
Bake in the middle of the oven for 40-45 minutes. After 20 minutes cover the tin with foil (if the top is already is browning). After 40 minutes poke the cake with a skewer - if it comes out clean, it’s done; if not, bake it for another 5 minutes or so.
When the cake is done, take the tin out of the oven. Leave the cake in the tin for 5 minutes, then transfer it to a wire rack. Let it cool completely before slicing.