Mix the sea salt and 200g of the sugar in a large food bag, add the pork and coat it well. (If you don’t have a bag, rub over the pork in a dish and cover with cling film.) Leave in the fridge overnight.
The next day, remove the pork and wipe down the meat with kitchen paper. Heat oven to 140C/120C fan/gas 1. Mix the remaining sugar, the maple syrup, mustards and some ground pepper. Rub half the mixture over the pork and sit it on a rack in a roasting tin. Roast for 6 hrs.
Spoon the remaining maple mixture over the pork and roast for 1 hr more.
Rest the meat for 30 mins on a plate loosely covered with foil. To serve, tear the pork into big fat chunks and, after skimming the surface, spoon over any juices from the tin.