Lightly grease a baking sheet. Keep aside.
Bring a saucepan of water to a rolling boil and add the potatoes, cook till tender. Reserve 1 cup of this water and discard the rest. Mash the boiled potatoes well. Keep aside till required.
In a large bowl, mix the yeast, flours, salt and caraway seeds together. Rub in the butter till the mixture resembles breadcrumbs. Add about 2/3rd cup of the reserved (and slightly cooled) potato water and the mashed potatoes and mix everything well and work into a dough. Add the rest of the water only if required.
Tip the dough onto a clean, lightly floured working surface and knead gently for about 7-8 minutes till pliable. Then place the ball of dough into a lightly oiled bowl and cover with an oiled clingfilm. Leave the bowl in a warm place till the dough has doubled.
Turn out the dough onto the working surface and gently punch out the air. Knead lightly and shape into a plump oval about 7 inches long.
Place it onto the prepared baking sheet and dust it with some wholemeal flour. Cover with a lightly oiled clingfilm and keep aside to rise, for about 30 mins
Preheat oven to 200 degrees C. Using a sharp knife slash the top of the dough with 3-4 diagnonal cuts (deep gashes) to make a criss cross pattern.
Bake for 30-35 minutes or till golden and sounding hollow when tapped on the base. Remove and allow to cool completely on a wire rack.
Slice and serve with butter, cheese or a savoury spread.