Dissolve baking soda in boiling water. Set aside.
In a very large bowl, cream together butter and sugar. Scrape bottom and sides of bowl as needed.
Add eggs one at a time and mix until incorporated.
Add buttermilk slowly and mix well.
Add soda water slowly and mix well.
Add the following ingredients, mixing well between each addition: white flour, whole wheat flour, salt, all bran, bran flakes, raisins and walnuts.
Pour into an airtight container and place in the refrigerator. Will keep for 2-4 weeks.
When ready to bake a fresh batch of muffins, preheat oven to 400°. Line a muffin tin with paper liners. Do not stir down the batter. Using an ice cream scoop, fill the muffin tin ¾ full. Bake until golden, about 15 minutes.