Use a baking tray; lightly grease the tray. Preheat oven to 220 degrees C. Sift flour, salt, mustard and baking powder into a bowl. Rub in butter with fingertips until the mixture resembles fine breadcrumbs.
In another bowl, stir the milk into the mashed potato, mixing well. Add to the dry ingredients and stir with a fork to mix; add another 1–2 tablespoons milk, if needed, to make a soft dough.
Turn the dough onto a lightly floured work surface; knead for a few seconds until smooth; roll out to a 15cm round about 2cm thick. Place on the baking tray. Using a sharp knife, mark the scone round into 6 wedges.
Brush with milk or beaten egg, then sprinkle with the oatmeal. Bake for 15–20 minutes or until well risen and golden brown.
Transfer to a wire rack; break into wedges. Serve warm or leave to cool. Scones can be kept in an airtight tin for up to 3 days. To reheat before serving; place on a baking tray, cover with foil and warm in a low oven for 5 minutes.