Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line a 2lb loaf tin with the oil. Mash 4½ bananas until chunky.
Put the sugar, eggs, mayonnaise and vanilla bean paste in a large bowl and whisk together until all the ingredients are well combined. Add the mashed bananas.
Mix together the plain flour, bicarbonate of soda, baking powder and cinnamon and add to the wet ingredients. Gently fold together until all the flour is incorporated and you have a smooth batter. Pour into the prepared tin. Place the reserved banana down the middle of the cake, cut side up. Dust with 2 tsp sugar.
Bake in the oven for 50-60 minutes, until well risen and a skewer inserted in the middle comes out clean. Take out of the oven and brush with the agave to glaze. Leave to cool in the tin for 15 minutes, then remove and leave to cool completely on a wire rack.