Mix the starter ingredients together in a bowl. Turn out and knead for 10 minutes. Place in a lightly oiled bowl and leave to rise overnight in the fridge.
To make the second dough, mix the remaining flours, salt and yeast in a large bowl. Make a well in the centre, stir together the butter and treacle, then pour in the dry ingredients along with almost all the water. If the dough seems a little stiff, add the rest of the water. Turn out onto a lightly floured work surface. Knead the starter into the dough and knead for 10 mins. Put the dough into a lightly oiled bowl and leave to rise for 1 hour until doubled in size.
Turn out onto a lightly floured work surface. Press the dough out into a rectangle about 2.5cm thick and sprinkle over the figs and dates, then roll up like a Swiss roll, kneading to combine evenly. Cut into 20 pieces and shape into rolls. Leave for 1 hour to prove until doubled in size. Heat oven to 220C/fan 200C/gas 7. Bake for 18-20 mins, until the rolls sound hollow when tapped underneath. Leave to cool on a wire rack.