Sift plain flour into a bowl, mix in the cream of tartar and bicarbonate of soda, sugar and salt.
Add butter and rub in with fingers until mix ressembles fresh breadcrumbs.
Make a well in the middle pour in the milk. Use a fork to work it into the dry ingredients.
Finish by kneading lightly but hand until it forms a soft but firm dough. Pat or roll the dough into a solid shape about 1 1/2 inches thick.
Using a cutter or glass cut the dough into rounds and place them on a greased baking tray.
Glaze the tops with the beaten egg. Bake for 15-20 minutes until golden. (Half that time in the Aga roasting oven).