Heat the oil in a large, deep frying pan or flameproof casserole dish. Add the garlic, marjoram and chicken and cook over a medium heat for 5 minutes, turning the drumsticks to lightly brown all over. Transfer the chicken to a plate.
Add the leeks, carrots and celery. Cook for 5 minutes until the leeks are softened. Stir in the stock and replace the drumsticks, shuffling them among the vegetables. Reduce the heat, cover and simmer for 30 minutes until the drumsticks are tender. Use a slotted spoon to transfer the drumsticks to a plate.
Add the corn and cannellini beans and heat through for 1 minute. Then, using the slotted spoon, transfer the vegetables to a serving dish. Cover and set aside to keep hot.
Bring the liquid in the pan to the boil. Stir in the sugar, vinegar, chilli sauce, tomato sauce and mustard. Boil hard for about 5 minutes until it has reduced to a slightly syrupy, shiny sauce.
Put the drumsticks in the sauce and heat through, turning them to coat evenly. Serve the drumsticks on the vegetables, pouring over all the sauce.