The day before, mix the sea salt and 200g of the sugar in a large food bag. Add the pork and coat it well, then leave it in the fridge overnight.
The next day, remove the pork and wipe the meat down with kitchen paper to remove any excess sugar or salt. Rub 3 tbsp of the jerk spices all over and sit the meat on a rack, in a roasting tin. Leave to marinate a little in the fridge, then 30 mins before you start cooking, bring it out to room temperature.
Heat oven to 140C/120C fan/gas 1. Mix together the remaining 100g sugar with the pineapple juice, the remaining jerk spices and treacle, and rub half over the pork. Pour the other half into the tin and roast for 6 hrs. Baste the meat with the juices and roast for 1 hr more until the outside is charred and the inside tender. Rest the pork on a platter for 20 mins and pour any juices from the tin into a fat-separating jug.
Put the diced banana in a bowl with the onion. Stone, peel and dice the avocados and add these, too. Mix in the lemon and lime juice, chillies, coriander stalks and leaves, and some seasoning.
Heat the black beans in a microwave, and warm the tortillas following pack instructions. Add a couple of forks to the pork for shredding, discard the fat from the tin juices, and serve the rest alongside for pouring. Serve the pork with the wraps, beans, banana salsa, yogurt and shredded lettuce, if you like.