In a mortar and pestle or electric spice grinder, pulverise the fennel seeds and sea salt. Preheat the oven to 220 degrees C.
Using a very sharp knife, such as a Stanley knife, score the pork belly skin with close together parallel cuts. Try to cut just the skin and fat, not the meat. This will allow a lot of the fat to drain when cooking and make the crispy crackling.
Rub the meat on all sides with the salt and fennel mixture and the black pepper, taking extra care to see that it gets into the cuts in the skin. You can use a little more salt if you think you need it.
Place the pork belly on a rack in a roasting tray. The rack will allow the fat to drain away from the pork as it roasts. Place in the preheated oven and roast 15 minutes until the skin starts to brown, then turn heat down to 170 degrees C and cook a further 2 1/2 - 3 hours.
Remove the pork from the oven and let rest 10 minutes. Slice and serve!