Prepare yeast batter: mix 1 cup flour, the sugar, the compressed or dry yeast and warm liquid together in a large bowl.
Cover and set aside for about 20 minutes in a warm place, longer in a cool one until frothy.
Mix the mashed potatoes, oil and salt into the batter and add enough of the remaining flour to make a firm dough.
Turn the dough onto a lightly floured surface and knead until smooth and elastic.
It will take about 10 minutes by hand or 2-3 minutes with a food mixer and dough hook.
Shape the dough into a ball and place in an oiled plastic bag.
Leave to rise until doubled in size.
Turn the risen dough onto a lightly floured surface, punch down and knead for about 2 minutes until the dough is firm.
Divide into 2 pieces and shape to fit two well greased small 4 1/2 x 2 1/2 x 1 1/2-inch loaf pans.
Cover and let rise.
Bake in the center of a hot oven (450°F) for about 25 minutes.
The crust should be deep brown in color.
Turn onto a cake rack to cool.