Preheat the oven to FAN 180 degrees Celsius, Gas Mark 6 or 400 degrees Fahrenheit.
In a bowl, peel and put the four ripe bananas and mash with a fork to the desired consistency; if liked, leave some pieces of banana un-mashed for a sweet bite, but if not, completely mashed will work perfectly too.
Add the sunflower oil, water, sugar, and (optional) maple syrup to the mashed bananas and mix well until you achieve a uniform consistency.
Now, slowly start adding the self-rising flour; add a couple of tablespoons at a time and mix well with the fork until completely mixed, ensuring no knots form. Continue doing so until all the flour is completely incorporated.
Line a tin loaf with greaseproof baking paper and transfer the vegan banana bread batter into the tin.
Place the tin on the middle shelf in the oven and cook for about 50 minutes; keep an eye on it, as some ovens may need a bit less or a bit more time. To ensure the cake’s batter is thoroughly baked, use a tooth pick in the middle of the loaf and if it comes out clean, it is ready.
Once ready, remove the tin from the oven and allow it to cool down as much as possible before removing it from the tin. Allowing it to cool down before slicing it will ensure the cake remains as moist as possible, as you won’t allow the steam to escape. You can store the uneaten slices of cake in an airtight container for up to three days.