Preheat the oven to 170°C/150°C Fan/gas mark 3/325ºF.Cut off the flap of fat that hangs over the duck's cavity. Sit the trimmed duck on a rack or slotted tray over a deep roasting tin and roast for 4 hours.Increase the oven temperature to 220°C/200°C Fan/gas mark 7/450ºF and give a final 30 minutes of intense heat. Or, if it's easier, just leave the duck in the low oven for 5 ½ hours. The choice is yours.Sit pancakes on the top part of a steamer to cook or simply follow the instructions on the packet.Pour the Hoisin sauce into a bowl.Cut the cucumber into matchsticks (like you get in a Chinese restaurant).Cut the spring onions lengthways into short strips (again, as in Chinese restaurants).Carefully take the duck, in its tin, out of the oven. Transfer the duck to a board, shred it, using two forks to pull the meat apart, and set it on the table with the sauce, pancakes, cucumber and spring onion shreds, so everyone can make their own pancake parcels.(When the duck fat has cooled a little, pour it into a bowl or jar, and save. It's great for frying potatoes.)