This is the easiest one-bowl banana cake recipe. Use a flat rectangular baking tin for this, as it will cook  more evenly.

4 bananas
11 oz superfine sugar (caster sugar)
4 large eggs
8 fl oz sunflower oil
15 oz plain flour
2 tsp baking soda (bicarbonate of soda)
2 tsp cinnamon
1/2  tsp salt
5 oz walnut pieces
4 oz raisins

Preheat the oven to 350 F
Mash the bananas until they are smooth. A potato masher is good for this.
In a large bowl, put the mashed bananas, and add all the other ingredients, one by one, stirring with each addition. It doesn’t matter in what order you put them in, except I recommend putting the raisins and walnuts in at the end.
Lightly grease your baking tin and pour the cake mixture in.
Bake for 1 hour.