To make the starter: Boil the potatoes in 250ml water until soft. Cool to lukewarm. Mix in flour and sugar. Pour into glass jar and cover with a clean cloth. Stand in a warm place until fermented. Let sit for at least overnight. When fermented, strain and reserve liquid.
Preheat oven to 230 C / Gas 8.
To make the bread: Sift flour, bicarbonate of soda and salt. Make a well in the centre. Pour in the strained potato liquid. Mix well and turn out onto a floured surface. Knead for 10 minutes, adding a little water if the mixture is too firm.
Place dough into greased loaf tin. Bake in preheated oven for 45-50 minutes.