Put gammon joint into slow cooker and add the cola, sugar, star anise and black peppercorns. Top up with water if needed. Cook on high for 4hr. Whisk together the treacle, mustard powder and honey until fully combined. Once the ham is cooked, drain and discard the cola and allow the ham to cool. Preheat oven to 200°C (180°C fan). Once the ham is cooled remove the skin, leaving some of the fat behind and score it. Brush over the glaze mixture and put into oven. Cook for 25-30min until golden brown. Keep brushing the glaze over regularly until it is all used up. Remove from the oven and allow to cool before slicing.