Place the flour, yeast and salt in a large bowl. Make a well in the centre and pour int he water. Using your hand to mix, bring the ingredients together to form a soft, pliable dough. Turn the dough out onto a lightly floured surface and knead vigorously for 5-10 minutes until the dough is smooth and stretchy.
Rub a little oil over the surface of the dough, cover with a clean tea towel and leave in a warm place for about an hour or until doubled in size.
Meanwhile, cook the potatoes in boiling water for 10-15 minutes until tender. Drainw well and mash with the milk. Stir in the cheese, onion, basil, salt and pepper.
Preheat the oven to 220C/450F/gas 7. Divide the dough in half and roll out each half into very thin 30cm rounds. Spoon the potato mix onto one half of each of the rounds. Dampen the edges and fold over the dough, pressing with your fingers to seal.
Place the calzone on a lightly oiled baking sheet. Bake for 15-20 minutes until crusty and golden.