Preheat the oven to 180°c (160 fan/ gas 4/ 350F) and line a 2lb loaf tin with baking paper. Dust the blueberries in a tablespoon of flour to prevent them from sinking to the bottom of the cake and set aside.
Cream together the butter and sugars with a wooden spoon until pale and fluffy.
Beat in the eggs and vanilla extract.
Fold in the flour.
Stir in the mashed bananas, fresh blueberries and mix gently until combined.
Spoon into the loaf tin and bake for 1 hour. Leave to cool in the tin before turning the cake out onto a cooling rack. Because ovens vary, start checking the cake after 50 minutes (use a skewer and see if it comes out clean).
