Pour the flours into a large bowl with the sugar, salt and yeast and mix well. Form a well in the centre and pour in the water, then mix the liquid around with your fingers, drawing flour in and gradually combining the ingredients to form a dough.
Transfer the dough to a floured surface; knead for 5 minutes, then put back in a clean bowl, cover with a warm, damp cloth and leave somewhere warmish to prove.
Preheat the oven to 200C/gas 6. Meanwhile, lightly oil a 20cm x 30cm baking tray. When the dough has doubled in size, which should take about 30 minutes, tip it back onto the floured work surface and knead again briefly, adding flour if it’s a little too sticky.
Rub a rolling pin with flour and roll the dough to form a 30cm x 20cm rectangle. Using the rolling pin, lift the dough and unroll it over the tray, dropping it neatly into place. Leave to prove for another 30 minutes.
When the focaccia has risen again slightly, push the chestnuts and rosemary springs into the dough. Lay the pancetta on top, so it can crisp up in the oven. Sprinkle with a little salt and drizzle with olive oil. Bake for 15–20 minutes, then remove and place on a cooling rack. Drizzle the focaccia generously with olive oil, and sprinkle with a little more sea salt.