Heat 5-6 tablespoons of the water until lukewarm then mix in the sugar and yeast.
Leave for 10 minutes until frothy.
Gently heat the molasses and margarine in a saucepan with the remaining water until melted, then stir in the vinegar and leave until lukewarm.
Place the wholewheat flour, bran, caraway and fennel seeds in a bowl with the salt; add the yeast liquid and molasses liquid and mix together to give a soft mixture; beatwell.
Work in the rye flour to give a fairly soft dough and then turn it out onto a lightly floured surface.
Cover the dough with the bowl and leave to rest for 15 minutes before kneading well for 10-15 minutes using as little extra flour as possible.
Place the dough in an oiled plastic bag and leave in a warm place until it has doubled in size.
Punch down the dough, knead until smooth and then shape into a round about6 inches in diameter.
Place the round on a greased baking sheet and let rise in a warm place until doubled in size, about 1 hour.
Bake in the center of a moderate oven (350°F) for 45-50 minutes.
Meanwhile combine the cornstarch and the cold water and cook over a medium heat, stirring constantly, until the mixture thickens.
Boil for 1 minute, stirring constantly.
As soon as the bread is baked, brush the cornstarch mixture over the top of the loaf.
Return the bread to the oven and bake for a further 2-3 minutes or until the glaze is set.
Remove the bread from the baking sheet and cool on a cake rack.