Put the gammon in a large pan and add the onion, carrots, peppercorns, cinnamon sticks and bay leaves. Pour over the ginger beer and top up with enough water to cover the gammon. Bring to the boil, skimming off any impurities. Simmer for 1 hour and 15 minutes, topping up with boiling water if the pork starts popping out. Remove from the heat and leave to cool in the liquid for 1½ hours.
Preheat the oven to 190°C/gas mark 5. Remove the gammon and transfer to a board. Remove the skin and score the fat into diamonds.
In a small saucepan, heat the honey, sugar, cinnamon and orange juice over a low heat, then increase the heat and simmer until it becomes syrupy.
Line a roasting tin with foil and transfer the gammon on to it. Add the quartered oranges. Brush half the glaze over the fat of the gammon then roast in the oven for 45 minutes, brushing the remaining glaze over it halfway through.