Preheat the oven to 150°C (300°F / gas mark 2) and put a 5 litre (20 cups) ovenproof dish with a lid in the oven.
Heat the butter in a saucepan over medium heat, stir in the leek and cook for about 2 minutes, then add the chicken and stir for 3 minutes. Toss in the rice and stir for 1 minute. Add the wine and stock and bring to the boil.
Pour into the oven proof dish and cover. Cook in the oven for 30 minutes, stirring halfway through.