Put the flour in a mixing bowl or a food processor. Add the butter and rub in with your fingertips or process until the mixture resembles fine breadcrumbs. Stir in the sugar, sultanas and orange rind.
Add all but 1 tablespoon of the egg then gradually mix in enough of the milk to mix to a soft but not sticky dough.
Knead lightly then roll out on a lightly floured surface until 1.5 cm (¾ inch) thick. Stamp out 5.5 cm (2¼ inch) circles using a plain round biscuit cutter. (Don't be tempted to roll out the dough thinner and make more scones as they will just look mean and miserly.) Transfer to a lightly greased baking sheet. Reknead the trimmings and continue rolling and stamping out until you have made 10 scones.
Brush the tops with the reserved egg and sprinkle lightly with a little extra caster sugar. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–12 minutes until well risen and the tops are golden. Leave to cool on the baking sheet.
Serve the scones warm or just cold, split and filled with jam and clotted cream. They are best eaten on the day they are made.