Mix the flour and baking powder together in a mixing bowl or a food processor then add the crushed seeds, lemon rind and sugar. Add the butter and rub in with your fingertips or process until the mixture resembles fine breadcrumbs.
Stir in the sultanas then the egg and enough milk to mix to a soft but not sticky dough.
Knead lightly then roll out on a lightly floured surface until 5 mm (¼ inch) thick. Stamp out 7.5 cm (3 inch) circles using a fluted round biscuit cutter. Transfer to a greased baking sheet. Reknead the trimmings and continue rolling and stamping out until all the dough has been used.
Prick the biscuits with a fork then sprinkle with a little extra caster sugar and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 8–10 minutes until pale golden. Transfer to a wire rack to cool. Store in an airtight tin for up to 5 days.