Set the oven to 200 fan. To rise well, scones need a really hot oven.
Grease a baking sheet and heat in the oven while you make the scones.
Sieve the flour, baking powder, custard powder and salt into a wide bowl.
Cut the butter into small pieces and rub into the flour mixture with your fingertips until it disappears.
Stir in the caster sugar, sultanas and orange zest.
In a separate bowl beat the milk, cream or yoghurt and eggs until combined and bubbles are appearing.
Make a well in the centre of the flour and pour in two-thirds of the eggs mixture. Using a knife, combine to make a soft dough; add more liquid if necessary.
Knead the dough into a bowl for a few turns to make a ball. On a lightly floured work surface, continue to knead to give a smooth surface, then roll out to about 3cm thick.
Dip the cutter into some flour to prevent it from sticking; cut out scones, re-flouring the cutter as needed, until the dough is used up.
Take the hot baking sheet from the oven and place the scones on it, a little apart. Brush them with the remaining egg mixture.
Bake for 15-20 minutes. The scones should be well risen and golden on the top and the bottom. Break a scone open to see if it is baked. If it's damp, bake for a further 3 minutes.
Remove from the oven and place on a cooling rack.
Serve hot, split and spread with butter, or store in an airtight container for up to two days. Reheat in a warm oven to soften them before eating.