Preheat oven to 180°C. Use a small sharp knife to remove the ham skin, leaving a section at the neck with a decorative edge. Score the fat in a cross-hatch pattern, then stud with cloves. Arrange in a roasting pan.
Combine the marmalade, brown sugar, vinegar and allspice in a medium saucepan and set over moderate heat. Boil for 5 minutes, until thickened. Brush half onto the ham, then bake for 45 minutes. Brush the remaining glaze onto the ham and bake for a further 45 minutes.
Meanwhile, steam the potatoes until tender, then crush them while hot. Toss gently in extra virgin olive oil and season with salt and pepper, then bake for 30 minutes. Add the agave syrup and butter, then bake for a further 20 minutes.