Place the fillet of ham in a large saucepan, cover with water and add the peppercorns, bay leaf and onion. Cover with a lid and simmer for 20 minutes per ½ kg and 20 minutes over - 1 hour 40 minutes for a 2kg joint.
Fifteen minutes before the joint is ready, preheat the oven to Gas Mark 6, 200°C (400°C).
Mix all the ingredients for the glaze in a bowl. Remove the joint from the water and place in a baking dish lined with parchment paper or tin foil. Allow to cool a little. Keep the cooking liquid to add to soups but discard the flavourings. Remove the layer of skin and score the fat. Brush half the glaze over the meat. Put the joint in the oven and cook for 20–30 minutes or until the top begins to brown. After the first 10 minutes brush over the remaining glaze.
To make the champ: Melt the butter in a small saucepan and gently sauté the scallions until softened. Pour in the milk and bring to a simmer. Mash the potatoes and then using a wooden spoon, beat in the milk mixture until you have achieved smooth, creamy mash. Taste and season.
To make the redcurrant sauce: Sieve the redcurrant jelly into a small bowl. Add the orange and lemon juice, port, ginger and mustard powder. Whisk to combine.