Heat oven to 200C/180C fan/gas 6 and grease a baking sheet. Put the flours, oats, bicarb and 1 tsp salt in a large bowl and stir to combine. Tip the yogurt into a jug, add enough water to make it up to 400ml and mix well. Stir the treacle and honey into the yogurt mixture until they dissolve. Pour onto the dry ingredients and stir with a round-bladed knife until you have a soft, sticky dough.
Tip onto a lightly floured surface and lightly form the dough into a round. Lift onto the baking sheet, cut a deep cross in the dough and sprinkle with oats. Bake for 30 mins or until the crust is golden and the loaf sounds hollow when tapped underneath. Soda bread should be eaten the same day, or toasted the next.