Prepare the dough by sifting and then mixing 150g of the rye flour with the plain flour and yeast. Make a well in the centre of the dry ingredients and then pour in half the water to make a thick paste. Beat together well for 5 minutes.
Cover and leave in a cool place (not the fridge for 12-14 hours) until it doubles in size.
Gently punch down the dough with a clenched fist. Add the remaining rye flour and the butter to the dough and the remaining water, and beat together to make a soft dough. Gradually add the water as the dough should not become too sticky.
Knead the dough for 3 minutes in the bowl and then tip it out onto a floured surface. Knead the dough on the work surface punching any air out of it, for another 3-4 minutes. Place the dough back into the bowl and leave in a warm place to double in size again. This should take about an hour.
Roll the dough into a cylinder pressing out any large air bubbles and slash the top diagonally with a knife then place on a lined baking tray for an hour and allow to rise and double in size again.
Preheat the oven to 220°C / gas mark 7.
Sprinkle a little rye flour on the top and then cook in the oven for 30 minutes. It should be golden brown and firm to the touch.
Allow to cool and then serve with a hearty casserole or stew.