Preheat the oven to 200oc/Gas 6/fan oven 180oc. Line a muffin tin with 12 paper cases. Sift the flour and baking powder into a large bowl, then stir in the sugar, ornage rind and salt. Beat the eggs and milk together in a jug or bowl, then stir in the butter and gently mix into the dry ingredients to make a loose, slightly lumpy mixture. Do not overmix or the muffins will be tough.