Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease a 33cm x 22cm rectangular cake tin and line the base and sides with baking paper.
Mix the oil, cake mix, water and eggs gently together in a bowl, then whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy. Whisk in the mashed bananas. Pour into the prepared tin and gently level the surface. Bake for 25-27 minutes until risen and golden and a skewer inserted into the cake comes out clean. Remove from the oven and cool in the tin for 5 minutes.
Using a metal skewer, poke holes in the cake at 1cm intervals, wiping the skewer clean occasionally to prevent sticking. For the tres leches, mix the condensed milk, coconut milk and cream together in a bowl. Setting aside 150ml of the mixture, spoon the remainder over the warm cake, spreading back and forth with the back of a spoon until absorbed into the holes. Leave in the tin to cool completely; about 1 1/2 hours.
To serve, remove the cake from the tin using the baking paper. Slice into 15 servings (see tip). Place each piece on a serving plate and drizzle over 1 teaspoon of the reserved milk mixture. Top each serving with a spoonful of icing, a couple of banana slices and a sprinkling of toasted coconut.