Toast the dried chillies in a large pan over medium heat until puffed and lightly browned in spots (about 5 minutes). Place in a bowl with 1 cup of warm water and stand for 15 minutes.
Place spices, dried oregano, chipotle chilli and their sauce, garlic, chilies with their soaking liquid, vinegar, honey and a good pinch of salt into a blender and blend until smooth.
Use a sharp knife to score the lamb shoulder on the fattier side. Rub the marinade all over the meat including into the incisions. Marinate in the fridge for a few hours or even better overnight.
Preheat the oven to 160°C. Place the lamb in a deep roasting dish, scored side up and add stock. Cover the tray with baking paper and then foil and roast for 2 - 2 1/2 hours before removing the baking paper and foil and cooking for a further 1 hour or until the meat falls off the bone easily and has a lovely crust.
For the salsa, char the corn either on the bbq or a grill pan, until blackened slightly. Cool and then slice the kernels off the cob and mix in a bowl with coriander, lime juice, salt and pepper.
Warm tortillas in a pan over a medium heat for a minute on each side. Keep warm in a tea towel.
To serve, place lamb on platter (serve with two forks, so meat can be shredded off the bone), along with lettuce leaves, the salsa and the warm tortillas for everyone to assemble themselves at the table.