Grease and line the base of four 200ml dariole moulds. Sift flour into a bowl, then add breadcrumbs, suet, sugar, sultanas and peel. Combine egg and milk, add to dry ingredients and beat well to combine. Spoon into moulds. Make a pleat in the centre of a sheet of baking paper (to allow for expansion) and use to cover pudding. Secure with string, then cover with foil. Place moulds in a saucepan, add water to come halfway up the sides, then bring to the boil. Cover, turn heat to low and steam for 1 hour.