Preheat the oven to 230 degrees C. Prepare a baking tray by lining it with non stick parchment paper.
Measure the flour into a big bowl. Sprinkle in the salt and, using a tiny sieve, sieve in the bicarbonate of soda so that it does not "ball up" in the dough. Use a big spoon to mix the dry ingredients together well.
Make a well in the dry ingredients and pour in the sour milk (or equivalent). if you want to add raisins, or cheese or anything else, add it now.
Using a spoon or a plastic scraper (or both) mix the ingredients together lightly but thoroughly. At first the mixture seems dry but you will see as you mix that it is really quite sticky. You may have to get your hands into the dough to get all the flour incorporated.
Wet your hands with water and pick up the dough and shape it into a ball.
Place the ball on a your baking tray and, with a wet hand, gently flatten the ball into a disc about 1.5 inches thick.
Using a wet knife or a wet scraper, cut a deep cross in the dough - almost all the way through the dough to the bottom - and gently open the cuts up a wee bit.
Put the dough immediately into the oven and bake for 10 minutes. Then, turn the oven down to 200 degrees C and bake it for a further 15-20 minutes. You don't want it dry but you do want it cooked so give it a little prod where the cuts separate the four quarters. If it is still squashy and clearly unbaked put it back for another 3-4 minutes.
Remove from the oven and then let it cool completely on a wire rack. Soda bread is best eaten the day it is baked.