Preheat the oven to 180°C (350°F/Gas 4). Grease and line the base of a 6 × 13 × 23cm (2½ × 5 × 9inch) loaf (bar) tin.
Sift together the flour, mixed spice, bicarbonate of soda and ¼teaspoon salt into a large bowl. Stir in the sugar and make a well in the centre.
Combine the eggs, butter, milk and banana. Add the banana mixture and sultanas, if using, to the flour well and stir until just combined. Pour into the tin and smooth the top. Bake for 45 minutes, or until the top is nicely coloured and a skewer inserted into the centre of the bread comes out clean. Cool in the tin for 10 minutes, then turn out onto a wire rack. Dust with icing sugar.