To make the crumpets, place the yeast into a bowl and whisk in the warm water.
Place the sifted flour in a separate bowl with the salt. Add the yeast mixture to the flour and whisk together until smooth.
Cover the bowl tightly with cling film and leave to prove in a warm place for one hour or until at least doubled in volume.
Preheat the oven to 170C/340F/Gas 3.
Heat a thin layer of light olive oil in a non-stick pan over a low to medium heat.
Lightly oil the inside of four 9cm/3.5in diameter by 2cm/0.75in high metal chefs' rings. Sit them inside the non-stick pan.
Pour or ladle enough of the proved batter into each of the four chefs' rings to come halfway up the sides.
Cook the crumpets in the chefs' rings in the pan until the top has completely set, then remove the chefs' rings and turn over. Cook for one minute until golden.
Transfer to a baking tray, place in the centre of the oven and cook for a further five minutes.
Remove from the baking tray and leave to cool on a wire rack until ready to serve.
To prepare the finished dish, preheat the grill to medium.
Place the vinegar and olive oil into a bowl and whisk together. Season, to taste, with salt and freshly ground black pepper. Set aside.
Heat a frying pan until hot and add 25g/1oz of the butter and the pancetta and fry for about three minutes until crisp.
Add the mushrooms, season with salt and fry for 3-4 minutes or until the mushrooms are golden brown. Add the chopped parsley and a grind of black pepper.
Meanwhile, heat a second frying pan until hot and add a little light olive oil until just warm.
Crack the duck eggs into the pan, making sure that the yolk is in the centre, and cook gently until the whites are fully cooked.
Place the pan under the grill for one minute to warm the yolks.
Meanwhile, toast the crumpets until crisp and golden brown.
Using an 8cm/3in plain-edged pastry cutter, cut the duck eggs into neat rounds. Season them with a little black pepper and sea salt.
Butter each of the toasted crumpets with the remaining butter and place them onto serving plates. Place a duck egg on top of each crumpet.
Spoon the mushrooms and pancetta on top of the eggs and drizzle with some of the dressing. Serve.