Pre-heat the oven to Gas Mark 6, 200°C, fan 180°C. Grease a baking tray with 15ml of the vegetable oil and set to one side.
In a large mixing bowl, sift together the flours, salt, baking powder and the caster sugar. Rub the margarine into the flour mix until they are no longer distinguishable. Create a well in the centre and pour in the rest of the vegetable oil and the buttermilk (make sure you reserve 1 tbsp of the buttermilk).
Use a spatula to bring the dry and wet ingredients together gradually until you have a moistened dough. Turn the dough out onto a lightly floured surface and knead gently for a minute until smooth. Shape into a loaf and arrange on the greased baking tray. Create a cross in the top of the loaf using you finger and brush with the remaining tbsp of buttermilk and dust with a little strong bread flour.
Bake in the centre of the oven for 30 minutes at Gas Mark 6, 200°C, fan 180°C and then reduce the heat to Gas Mark 4, 180°C, fan 160°C, turn the loaf and bake for a further 30 minutes. Remove from the oven and allow to cool on a wire rack before slicing and serving with butter.
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