Mix the marinade ingredients together in a bowl, then add the duck. Using your hands, coat the duck in the marinade. Cover with cling film and refrigerate for 20 minutes. Soak 12 wooden bamboo skewers in cold water.
Meanwhile, make the pancakes. In a large bowl, mix 600g/1lb 5oz plain flour with all the strong bread flour and the salt. Add the boiling water and mix into a dough. Add 150ml/5fl oz cold water and knead into a smooth dough. Set aside.
Heat the rapeseed oil and sesame in a small pan and add the white pepper, black pepper and salt. Remove from the heat.
Roll out the dough into a flat circle and spread the seasoned oil onto it, taking care not to spread it right to the edges. Scatter the spring onions onto the dough, then scatter the remaining plain flour over the top.
Cut the dough into eight sections, leaving each section attached 2cm from the centre. Take one section and fold towards the centre of the circle, then repeat with the remaining sections. Roll out again and repeat the circular folding. (This creates stretchy chewy layers in the pancake).
Roll out the dough again into a rectangle and then roll into a long Swiss roll shape; it should be about 10cm/4in wide. Divide into six pieces and divide each piece into two. Flatten each piece with your palm and use your hands or a rolling pin to stretch the dough into a circle. You should end up with 12 small pancakes.
Heat a wok or pan over a medium heat. Add a tablespoon of rapeseed oil and swirl it up the sides of the pan (alternatively you can brush the pancakes with some rapeseed oil on each side to ensure even cooking). Add two to three pancakes and cook until golden-brown, turning halfway. Repeat until all the pancakes are cooked. Cover with foil and keep warm in the oven until ready to serve.
To cook the skewers, heat a griddle pan over a high heat. Add 2 tablespoons rapeseed oil to the duck and mix it through the meat. Thread the meat on to the bamboo skewers, dividing it equally between the skewers.
Just before adding the skewers to the pan, rub a little more oil onto them. Cook for 1 minute, then when they no longer stick to the grill, turn and cook for a further minute. Transfer to a board.
To serve, lay the pancakes, skewers and all the accompaniments out on a serving board so people can assemble the pancakes themselves. Spread hoisin sauce on a pancake, top with lettuce, then the duck, the sweet chilli mayo, the sriracha, and finally the cucumber.