Place the achiote paste, vinegar, onion, garlic, herbs, salt, spices and olive oil in a blender and pulse to a paste. Slowly pour in the orange juice with the motor running to incorporate into the paste. Pour the marinade all over the pork ensuring it is thoroughly coated and then marinate overnight in the refrigerator.
Preheat oven to 150C/130C fan/gas 2 Transfer the pork belly and its marinade to a large casserole and cover with foil or a tight-fitting lid. Cook slowly for 4 hours or until the pork is soft and falling apart to the touch.
Serve the pork pibil covered with sauce with boiled potatoes or Shred the pork using 2 forks, discarding the fat to make a delicious fajita garnished with sliced red onions, chopped green chillies and sprinkled with Mexican spice mix.