Preheat the oven to 220°C (425°F/Gas 7). Lightly grease a baking tray orline with baking paper. Sift the flour, baking powder, mustard and a pinch of salt into a bowl. Using your fingertips, rub in the butter until the mixture resembles fine breadcrumbs. Stir in the parmesan and 60g (2¼oz/½cup) of the cheddar cheese, making sure they don’t clump together. Make a well in the centre.
Add almost all the milk and mix with a flat-bladed knife, using a cutting action, until the dough comes together in clumps. Use the remaining milk if necessary. With floured hands, gently gather the dough together, lift out onto a lightly floured surface and pat into a smooth ball. Do not knead or the scones will be tough.
Pat the dough out to a 2cm (¾inch) thickness. Using a floured 5cm (2inch) cutter, cut the dough into rounds. Gather the trimmings and, without over-handling, press out as before and cut more rounds. Place the rounds close together on the tray and sprinkle with the remaining cheese. Bake for 12–15 minutes, or until risen and golden brown. Serve the scones warm or at room temperature.