Mix both the flours, salt, and yeast together in a large bowl. Add the butter and rub it in with your fingertips. Stir in 5 tablespoons of the seeds. Make a well in the centre of the flour mixture and pour in 300ml hand-warm water. Mix everything together to make a dough. Gather the dough into a ball.
Turn the dough out onto a lightly floured surface and knead for 8 to 10 minutes or until smooth and elastic. Put the dough in a large, lightly oiled bowl and cover with cling film. Leave to rise in a warm place for about 1.5 hours or until the dough doubles in size.
Line a large baking sheet with baking parchment. When the dough has risen, gently knead it on the lightly floured work surface 3 or 4 times (no more, as too much handling now will reduce the dough’s lightness).
Cut the dough into 2 equal-sized portions and roll the pieces into “ropes” about 40 cm long. Lay one on top of the other to make an “X”, then twist them together tightly. Tuck the ends under and press to seal.
Lay the loaf on the prepared baking sheet and cover with a clean tea towel. Leave for 40 to 50 minutes or until doubled in size. Preheat the oven to 230C (fan 210C/450F/Gas 8).
Uncover the loaf and brush the top with cold water. Sprinkle the remaining 1 tablespoon of seeds over the twists and gently press in. Bake the loaf for 10 minutes, then reduce the oven temperature to 220C (fan 200C/425F/Gas 7) and bake for a further 20 minutes.
When the loaf is baked it should be golden brown and firm. I tap it on the base to check if it’s ready. It should have a hollow drum-like sound. If it doesn’t, return the loaf to the oven for a few more minutes. Place on a wire rack to cool.