Preheat the oven to 200°C (400°F). Place the lamb heads and pork trotters in a roasting tin and roast, turning once, until dark brown. Transfer everything, including the pan juices, to a large heavy-based pan. Add the chicken feet, leeks, onions and 1.5 litres (52 fl oz) water and cook, uncovered, for 12 hours over low heat. Strain the liquid through a fine strainer, put in the fridge and leave to cool.
Remove the excess fat from the top and put the stock in a heavy-based pan. Add the sherry vinegar and then cook over medium heat for 3–4 hours until reduced to a clear, gelatinous glaze.