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Heat oven to 160C/140C fan/gas 3. Scatter the onions and bay leaves in the bottom of a large roasting tin. Mix the mustard powder, paprika and 1 tsp ground black pepper with a good pinch of salt. Rub this all over the pork, making sure you rub it into all the crevices. Place the pork, rind-side up, on top of the onions. Pour 200ml water into the bottom of the tin, wrap well with foil and bake for 4 hrs. This can be done up to 2 days ahead – simply cover the tray in foil and chill until ready to barbecue.
Light the barbecue. In a bowl, mix the ketchup, vinegar, Worcestershire sauce and brown sugar. Remove the pork from the tin and pat dry. Place the roasting tin on the hob, pour in the ketchup mixture and bubble vigorously for 10-15 mins until thick and glossy. Remove the bay leaves and pour the sauce into a food processor; blitz until smooth. Smear half the sauce mixture over the meat.
Once the barbecue flames have died down, put on the pork, skin-side down. Cook for 15 mins until nicely charred, then flip over and cook for another 10 mins. The meat will be very tender, so be careful not to lose any between the bars. Alternatively, heat a combined oven-grill to high, place the pork on a baking tray and cook each side for 10-15 mins until nicely charred.
Lift the pork onto a large plate or tray. Remove string and peel off the skin. Using 2 forks, shred the meat into chunky pieces. Add 3-4 tbsp of the barbecue sauce to the meat and toss everything well to coat. Pile into rolls and serve with extra sauce and a little coleslaw.