Preheat the oven to 200°C (400°F). Place the lamb bones in a roasting tin and roast, turning once, until golden. Transfer to a large stockpot and add 5 litres (175 fl oz) water. Slowly bring to a simmer, skimming occasionally to remove any impurities. Allow the stock to simmer very gently, uncovered, for 10–12 hours. Strain through a fine strainer. Return to low heat and simmer until reduced by three-quarters. Add a splash of cream and then allow to boil until the lactose converts to caramel and the sauce is rich and glossy.