Preheat the oven to Gas Mark 8, 230ºC (450ºF). Sift the plain flour, salt and bicarbonate of soda into a large mixing bowl and stir in the wholemeal flour, 25g of the oats and 75g of the grated cheese. Make a well in the centre, add 450ml of the buttermilk and mix together to make a soft but not too sticky dough. Add a little more buttermilk or milk if the mixture seems a little dry. Turn the mixture out onto a lightly floured surface and knead very lightly and briefly into a round. Flip the dough over and gently flatten it into a disc about 4cm thick. Lightly dust a baking sheet with flour and place the dough on it and sprinkle over the remaining cheese and a few more oats. Then using a large, sharp knife, cut a large cross into the top of the loaf, almost all the way through the dough. Bake the loaf on the middle shelf of the oven for 15 minutes. Then lower the oven temperature to Gas Mark 6, 200ºC (400ºF).and bake for a further 20-25 minutes, until it sounds hollow when you tap the base. Leave to cool before serving.