Preheat oven to 220 degrees (Celsius). Tip 1kg self-raising flour, a teaspoon of cream of tartar, and 150g butter into the bowl of a food processor. Pulse the mixture until it looks like coarse sand. Scoop 200ml of light sour cream into a measuring jug, top up with soda water to 600ml, then pour both into the bowl with the flour mixture. Process until a dough just forms, then scrape it onto a lightly floured surface. Knead lightly, then pat the dough to an even thickness to match the height of your scone cutter. Cut into rounds and arrange close together on a tray lined with baking paper. Roll scraps of dough into a ball, knead lightly, and repeat the cutting process. Brush top of scones with milk. Bake in the preheated oven for 10-15 minutes until tops are nicely browned.