In a bowl, mix the flour, Stevia with the baking powder. Combine well.
Rub the margarine into the flour mixture until it resembles breadcrumbs.
Add the yogurt and raisins (if used) and mix gently with a spatula until it forms a soft dough.
Chill the dough in the fridge for about a hour.
Preheat the oven to 180°C.
Roll the dough out between two sheets of parchment paper to 2cm thickness, then stamp out round shapes, using a round cutter or mug.
Place the scones on a baking tray, lined with baking paper.
Bake them for 10-12 minutes until risen and golden on top.
Serve them warm or at room temperature.