Line a roasting pan with a large piece of foil which comes a long way over the edges of the pan. Using a very sharp knife, score the skin of the pork in a criss-cross pattern all over. Then insert the knife into the flesh of the pork in several places to make pockets for flavourings. Put the leg on the foil in the roasting pan. Peel the garlic and ginger and chop them finely together. Mix them in a bowl with the grated orange rind and 1 tablespoon of the mustard. Now press this mixture into the deep incisions you have made in the pork flesh. Then mix the orange juice with the remaining mustard, the honey and the ground spices in another bowl. Season with plenty of black pepper and rub the mixture all over the joint but not on the hard skin. Bring the foil up all round the pork but leave the hard skin exposed. Leave in a cool place overnight.
When you are ready to cook the joint, heat the oven to Gas Mark 5/375°F/190°C. Smear the hard skin with oil and sprinkle if with caster sugar and sea salt. Cover the joint loosely with greaseproof paper, tucking it into the foil. Cover the shank (thin) end completely with foil, so it does not dry out. Put the pork into the centre of the oven for 25 minutes per pound plus 25 minutes, opening up the foil and removing the paper completely for the last 30 minutes or so. If the hard skin hasn’t crisped you can turn up the oven to its highest setting for about 15 minutes at the end.