Set the oven to 200°C fan, 250C, 425°F, gas 7-8. 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 egg mixture. Using a knife, combine to make a soft dough; add more liquid as necessary.
Knead the dough in the 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 3 cm thick.
Dip the cutter into some flour to prevent it from sticking; cut out scones, re-flouring the cutter as needed, until all 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 remaining egg mixture.
Bake for 15-20 minutes; the scones should be well risen, golden, and the bottoms also golden. 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.