Preheat the oven to 140/275/gas 1. Take the gammon out of its packaging and place in a large casserole dish with a tight fitting lid. Add the peppercorns and bay leaf to the dish and completely cover with cold water. Heat to a simmer on the stove, scraping off any white froth which comes to the surface using a spoon. Discard the white froth.
Once the gammon has reached simmering point, put the lid on top and place it in the oven.
Cook for 2 hours for 1.5kg, 2.5 hours for 2kg and 3.5 hours for 2.5kg.
While the gammon is cooking, make the paste. Mix the ketchup, brown sugar and soy sauce in a small bowl and set to oneside. Once the cooking time is up, remove the gammon from the oven. Take off the outer skin and score the fat in 4cm crisscross diagonal lines, creating diamonds.
Turn the oven temperature up to 200/400/gas 6.
Spoon the sauce over the scored fat and the sides of the cooked gammon. Place the gammon in a roasting tray and bake for 20-25 minutes, until the sauce has hardened and has started browning.
Remove from the oven and leave to rest for 5 minutes before serving. If you’d like to prepare it in advance, boil the gammon in the morning or the day before and simply bake it 30 minutes before you are ready to eat.