Preheat the oven to 180C/350F/Gas 4. Line eight holes of a muffin tin with paper muffin cases.
Sift the flour, baking powder, cinnamon and salt into a large mixing bowl. Add the banana and blueberries and, using a fork, lightly toss the fruit through the flour.
In a separate bowl, lightly whisk together the honey, vegetable oil, egg and milk.
Add the liquid ingredients to the dry ingredients and stir until just combined (don't over-mix or your muffins won't be as light in texture). Spoon the batter into the individual muffin cases.
Mix the extra half a teaspoon of cinnamon and the caster sugar together and sprinkle on top of the batter.
Bake in the oven for 20 minutes, until risen and lightly golden. Serve warm from the oven.