In a measuring jug, combine the water (mix 1 part boiling water to 2 parts cold water to make up 300ml) with the sugar and yeast and allow to rest for 15 minutes until foamy.
When the yeast is foamy, put the flours into a bowl and add the yeast mixture. Stir with a wooden spatula, adding in a little more water as needed to bring it to quite a sloppy consistency. Add in the salt and crushed vitamin C tablets. Stir well then leave to soak for 15 minutes.
After 15 minutes, begin gradually adding more flour while kneading until a dough forms. Turn out onto a floured surface and knead and stretch the dough, repeating until the dough feels like a soft rubber ball. Put the dough into a bowl and leave in a warm place until it doubles in size. This should take a couple of hours.
After it has completed the first rising, punch it down a little and knead some more. Grease a 900g loaf tin and place the dough inside. Leave in a warm place for about an hour for second rising until it has doubled in size.
Preheat the oven to 220 C / Gas 7.
Bake in the oven for 20 minutes at the preheated temperature, then rotate the tin in the oven and lower the oven temperature slightly and continue to cook for a further 25 minutes. Once risen and golden, remove from the oven and allow to cool.