Preheat the oven to 190C/170C fan/Gas 5. Lightly grease two baking trays or line them with baking parchment.
Take a large mixing bowl and add all the dry ingredients. Mix together.
Dice or grate the butter then rub it into the dry ingredients until you have a mixture that looks like breadcrumbs.
Stir in enough of the beaten egg to make a soft dough, using your hands to bring the mixture together.
Generously sprinkle the work surface with oatmeal. Put the dough on the oatmeal, sprinkle more over the top then roll out 3-5 mm thick.
Using a round, 8cm cutter, cut out biscuits and transfer them to the greased baking trays. Gather up the scraps, re-roll and cut out more biscuits until all the dough is used up, dusting with more oatmeal as needed. Depending how thick you rolled the dough, you should get 12-15 biscuits.
Prick holes into each biscuit in a pleasing pattern using a skewer or cocktail stick.
Put the trays of biscuits in the preheated oven until cooked through and golden brown (15-20 min). Turn the trays around half way through if the biscuits are cooking unevenly. Transfer biscuits to a wire rack until completely cold before storing in an airtight container.