Preheat the oven to 250°C and let the gammon reach room temperature.
Line a roasting tin with foil, place a rack inside, and layer two large sheets of foil crosswise.
Place gammon on the foil and pour 150g of black treacle over the rind.
Seal the gammon tightly in three layers of foil to create a steam-tight parcel.
Bake at 250°C for 30 minutes, then reduce heat to 100°C for 12–24 hours.
Remove the ham from the oven, discard string and rind, but keep the fat layer intact.
Increase oven temperature to 200°C and score a diamond pattern into the fat.
Stud with cloves and coat with a mixture of treacle, sugar, and mustard.
Bake for 20 minutes until the glaze is blistered, then rest before carving.