Sift the flour into a bowl with the salt, bicarbonate of soda and sugar. Coarsely grate the butter into the mixture. Stir well to break up any clumps of butter, add the buttermilk or yogurt and mix to a soft dough.
Knead the dough lightly until smooth, shape into a ball and pat out with your hands to a large circle about 30 cm (12 inches) across. Place on a baking sheet.
Spread the topping sauce over the dough, leaving a small border around the edge. Mix together the chicken and barbecue sauce and spread over the pizza. Arrange the peppers and mozzarella on top and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes until the base is cooked and the topping is golden.