Preheat the oven to 180°C (350°F/Gas Mark 4).
Cut each muffin in half to make 4 rounds pizza bases. Place them cut-side up on a baking tray.
Spread a thin layer of tomato puree over the cut sides of the muffins.
Using a round cutter that is roughly the same size as the muffin, cut 4 circles from the red Leicester cheese slices. Use the same cutter to cut crescent slices from each side of the circles.
Arrange a cheese circle on the top of each muffin, leaving a small gap between the pieces to resemble the lines on a pumpkin.
Cut 4 Jack O'Lantern face shapes from the red pepper.
Arrange the pepper face shapes on the top of each pizza.
Bake in the oven for 5-10 minutes until the cheese is melted and starting to bubble.
Leave to cool for a few minutes before serving, or cool completely and pack in a lunch box.