Grease a 2 lb / 900 g loaf tin and line with greaseproof paper. Cream the fat and sugar together in a bowl or food processor until pale and light, then beat in the egg yolks and yoghurt. Sift the flour, baking powder and spices together, and stir into the cake batter. Add the nuts, carrots and bananas, mixing thoroughly. Whisk the egg whites until stiff, and fold into the cake mixture. Pour into the loaf tin, making a slight depression down the centre of the mixture to prevent it rising too much in the middle and cracking. Bake in the centre of a preheated oven at 180°C / 350°F / Mark 4 for about 1½ hours. Lay buttered paper or foil over the top if the cake looks as if it is browning too much. Leave to cool in the tin for 10 minutes or so after removing it from the oven. Turn on to a wire rack, and remove the paper. Slice when cold.