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Toast the spices in a dry frying pan for a couple of mins. Pound them together in a pestle and mortar with some flaked sea salt, the garlic and half the thyme to make a paste, then mix with 2 tbsp olive oil.
Lay the pork on a board skin-side down. Rub the herb mix all over the flesh then scatter with the remaining whole thyme leaves. Neatly roll the meat into a joint surrounded by the skin then use butchers' string to tie the joint tightly at regular intervals to hold the joint together. Cover and chill, leaving to marinate for a few hours or overnight.
When ready to cook, rub the skin of the joint with plenty of salt and 1 tbsp remaining olive oil. Put it on a wire rack and roast at 200C/180C fan/gas 6 for 30 mins. After this time, squeeze the lemons over the skin and turn the heat down to 180C/160C fan/gas 4. Roast for a further 2 hrs. Finally turn the heat back up to 220C/200C fan/gas 7 and give it a final blast for another 30 mins or so, to finish the crisping of the skin. Allow to rest somewhere warm for 20 mins. Carve up into thick slices and serve with your favourite roast dinner accompaniments.