Preheat the oven to Gas Mark 7/220°C/Fan200°C. Mix together the flours, yeast and salt in a large mixing bowl. Measure out 350ml (12floz) hand warm water. Make a well in the centre of the flour and pour in the treacle and enough water to make a soft, but not sticky dough, when you stir together. Turn out onto a lightly floured surface and knead for 10mins by hand until the dough is smooth and elastic. You can also knead in a machine for about 5mins.
Place in a bowl, cover with clingfilm and leave in a warm, draught-free place until the dough has doubled in size, about 1hr. Knead a couple of times to remove any large air bubbles. Grease a 1.2 litre loaf tin (about 23 x 13 x 5.5cm). Place the dough into the tin, pressing it out evenly. Loosely cover with an oiled piece of clingfilm and leave to rise again until the dough has filled the tin, about 45mins.
Lightly brush over with some water and then scatter with the seeds and oats. Bake in the oven for 20 mins. Then reduce the oven to Gas Mark 6/200°C/Fan180°C and bake for 15 – 20mins more until the bottom is hollow sounding when tapped. Turn out onto a wire rack and leave to cool.
Cut the bread into slices and serve with smoked salmon, butter and lemon wedges.