Preheat the oven to 190°C, fan 170°C, gas 5.
Mix together the plain flour, wholemeal flour, salt, bicarbonate of soda, cream of tartar and sugar in a large bowl. Add the butter and rub in, using your fingertips, until the mixture resembles breadcrumbs.
Stir in the buttermilk and enough milk (start with 100ml) to mix to a stiff consistency, taking care not to over-mix. Knead very lightly on a floured surface and shape into a ball.
Place the dough on a lightly floured baking sheet. Cut a 2.5cm-deep cross in the centre and bake in the oven for 25-30 minutes until risen and golden and the centre of the cross feels firm
Remove from the oven and wrap in a clean tea towel to cool.