Grind the fennel seeds and peppercorns using a mortar and pestle (or in a blender), then add the salt, paprika, cumin, allspice or cinnamon and chilli powder and mix well.
Rub the spice mixture over the pork, rubbing well into the fatty bits and leave covered in the fridge for at least 2 hours (up to overnight).
When ready to cook, rub the oil into the meat and sear it in a hot frying pan until brown all over.
Throw the meat into a slow cooker or oven-proof casserole dish along with the rest of the ingredients and cook on low (about 110oC) for about 8 hours or high (or 150oC) for about 5 hours.
Take the pork out, place it in a dish and use a fork to pull the meat into shreds.
Put the shreds back into the slow cooker or casserole with the sauce and cook for another 20 minutes, uncovered, on high. (To be honest, I don’t bother with this part I just dish up the meat with the sauce poured over the top!)