Make the marinade: combine thyme leaves, allspice, cinnamon, mustard, orange juice, garlic, oil and vinegar, blend into a paste using a small coffee grinder or mortar & pestle.
Season pork shoulder with salt on all sides. Rub the marinade into the meat evenly. Cover and transfer to the fridge for 6 – 8 hours or preferably overnight.
Remove pork from the fridge one hour before roasting.
Heat oven to 180°C. Place pork on a roasting tray and roast for 50 minutes.
Combine orange juice, marmalade, allspice and rum. Whisk to combine.
After 50 minutes remove pork from the oven and using a brush, glaze the pork with half of the mixture on all sides until nicely coated with the glaze. Return to the oven for another 10 minutes.
Repeat the process after 10 minutes, using the rest of the glaze and cook a further 10 minutes. The total cooking time including both glazing sessions is 1 hour and 10 minutes.
Remove pork shoulder from the oven and check that the meat is cooked through but still slightly pink inside (it will continue to cook as it cools down so it’s important not to overcook the meat). Allow the pork to rest for at least 20 minutes.
Transfer to a plate or platter and carve into thin slices for serving.