Mix the molasses and the buttermilk in a large bowl. Stir in the oats and then let soak for 30 minutes.

Preheat oven to 400 F.

In another bowl, mix the flours, baking soda and salt. Add the pieces of butter and rub in, using the tips of your fingers, until the mixture looks fine like crumbs.

Add the beaten egg and the mixture of soaked oats and mix well with a wooden spoon to make a fairly heavy dough. Scrape the mixture into the greased loaf pan and, with floured fingers, press it into a neat loaf shape.

Bake for 10 minutes, then reduce the oven temperature to 350 F and bake for a further 35 minutes.

Turn out the loaf onto a wire rack and let cool to room temperature before slicing.

Keep tightly wrapped and eat within 3 days, or toast. The loaf can be frozen for up to 1 month.