Preheat the oven to 190C, 170C Fan, 375F, or Gas 5. Line a baking tray with parchment paper.
Mix the two different types of oats, salt, and bicarbonate of soda.
Add the melted butter to the oats and stir until the oats are evenly coated. Add the water gradually until a thick dough forms.
Dust a clean side with flour and transfer the dough to the side. Roll the dough out using a lightly floured rolling pin until it's around 2-3cm thick. If the dough breaks apart at the edges, gently push it back together. If it seems too dry, add a splash of water.
Using a 6-7cm pastry cutter, cut out several biscuits and lay them on a baking tray lined with parchment paper.
Bring the leftover offcuts together and re-roll. If necessary, add a splash of water, as they will dry out as you start re-rolling. Repeat this process until you have no more dough left.
Transfer the baking tray lined with biscuits to the oven. Bake for 15 minutes. Turn the biscuits over and bake for another 10-15 minutes.
Allow to cool slightly on a wire rack.
