Brush a 10 × 26cm (4 × 10½inch) bread tin with oil. Put the barley in a saucepan with 500ml (17fl oz/2cups) water, bring to the boil and boil for 20 minutes, or until softened. Drain.
Put the yeast, sugar and 150ml (5fl oz) warm water in a small bowl and mix well. Leave in a draught-free place for 10 minutes, or until bubbles appear on the surface. The mixture should be frothy and slightly increased in volume. If your yeast doesn’t foam, it is dead, so you will have to discard it and start again.
Put the barley, linseeds, soy and gluten flours, wholemeal flour, 250g (9oz/2cups) of the plain flour and 1teaspoon salt in a large bowl. Make a well in the centre and add the yeast mixture, oil and 150ml (5fl oz) warm water. Mix with a wooden spoon to a soft dough. Turn out onto a floured surface and knead for 10 minutes, or until smooth and elastic. Incorporate enough of the remaining flour until the dough is no longer sticky.
Place the dough in a lightly oiled bowl. Cover with plastic wrap or a damp tea towel and leave in a draught-free place for 45 minutes, or until doubled in size. Punch down and knead the dough for 2–3 minutes.
Pat the dough into a 20 × 24cm (8 × 9½inch) rectangle. Roll up firmly from the long side and place, seam side down, in the bread tin. Cover the dough and leave in a draught-free place for 1 hour, or until risen to the top of the tin. Preheat the oven to 200°C (400°F/Gas 6).
Brush the dough with water and make two slits on top. Bake for 30 minutes, or until golden and the bread sounds hollow when tapped on the base. Remove from the tin and cool on a wire rack.