Cook the potatoes in boiling, salted water for 10 minutes or until tender. Drain thoroughly and return 150 g (5 oz) to the pan. Mash until smooth; let cool.
Lift the bread pan out of the machine and fit the blade. Put the ingredients, except the thyme springs, in the pan, following the order specified in the manual. Add the mashed potatoes with the milk. Fit the pan into the machine and close the lid. Set to the dough programme. Melt the remaining butter.
At the end of the programme, knead in the remaining diced potatoes. Divide the dough in half and shape into 2 long, slender loaves. Space slightly apart on a large, greased baking sheet. Cover loosely with oiled clingfilm and leave to rise in a warm place for 45 minutes.
Brush the loaves with half the melted butter and sprinkle with sea salt. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 25 minutes until risen and golden. Brush with the remaining melted butter and sprinkle with extra thyme leaves. Serve warm or cool.