Preheat the oven to 220℃/200℃Fan/425℉/Gas 7. Lightly grease a baking tray.
Measure the flour, salt, mustard powder, cayenne and baking powder into a bowl. Add the butter and rub it in with the fingertips until the mixture resembles fine breadcrumbs. Stir in 100g of the grated cheese.
Break the egg into a measuring jug and make up to 150ml with milk. Stir the egg and milk in the fry ingredients and mix to a soft but not sticky dough.
Turn out onto a lightly floured surface and knead lightly. Roll out to a 15 - 17 cm circle and mark into 6 wedges. Transfer to the baking tray.
Brush with a little milk and sprinkle the remaining grated cheese.
Bake in the oven for about 15 minutes or until golden brown and firm to the touch. Slide onto a wire rack to cool.
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